hen Larry Johnson applied for unemployment benefits the week after the World Trade Center collapsed, he seemed like just the kind of person government officials have been talking about helping since Sept. 11.

Mr. Johnson, 49, worked the night shift, cleaning the bar and buffing floors, at Windows on the World, the restaurant at the top of the north tower. The terrorist attacks, two hours after Mr. Johnson had left work, killed 79 colleagues and took away his job eight months after he had started it.

But a couple of weeks ago, he received a letter from New York State rejecting his application for benefits because he had not earned enough money over the previous 18 months. That technicality is one of the many restrictions in an unemployment system that now pays benefits to about 39 percent of all Americans who are without a job and looking for one, down from about 50 percent in 1975 and even higher levels a half- century ago, according to the Labor Department.

As the labor force has changed over the last three decades to include more women, more temporary employees and fewer factory workers, the unemployment insurance system has not changed with it in many states and has become considerably stricter in many others, job counselors and economists said.

The system, created in 1935 as part of the Social Security Act, pays an average of $230 a week to unemployed Americans who meet its qualifications. Although the rules vary from state to state, they typically exclude people who do not fit the traditional notion of a laid-off worker — part-time employees, recently hired low-wage workers and people who quit their jobs or refused a new shift for family reasons.

As a result, because millions of unemployed workers will not qualify for benefits in the first place, many laid-off employees will not be helped by President Bush's proposal to extend payments to a limited pool of those jobless Americans who are eligible.

With the United States probably in its first recession in a decade and companies announcing tens of thousands of layoffs, the gaps in the unemployment safety net could aggravate the downturn, economists say. People without jobs or unemployment benefits — even people whose spouses work — are likely to cut their discretionary spending sharply, as many struggle to pay their rent, meet their debt payments and still have enough money for food and transportation.

"I've got to start all over again," said Mr. Johnson, who described himself as running out of the money he had received from his sister and a few charities. "I'm on the edge of being homeless."

Mr. Johnson was turned down because New York, like most states, determines eligibility based on earnings over a 12-month period that ends a few months before the date a job is lost. This is supposed to give the state time to gather employment records, but it often excludes people who have worked for less than a year and were previously without a job.

New York is one of nine states that let people like Mr. Johnson reapply and ask that the most recent months be considered, said Betsy McCormack, a spokeswoman for the state labor department. With each rejection letter, the state includes a form allowing people to make the request.

In addition, Ms. McCormack said, Gov. George E. Pataki has taken steps to help people left jobless because of the attacks, like extending until next month the deadline to apply for disaster-related unemployment benefits.

Still, Mr. Johnson's unexpected rejection, and the difficulties it has caused, is typical of the effect that an increasingly complicated set of rules has had, job counselors said.

"This is a system that defies explanation in so many ways," said Jeffrey B. Wenger, an economist at the Economic Policy Institute, a research group in Washington partly supported by labor unions.

Full-time employees who worked for at least a year before losing their jobs rarely fail to qualify for benefits because of income minimums; most states require people to have earned only a few thousand dollars to be eligible.

But people are rejected for many other reasons. Some are turned down because they quit their jobs to care for relatives, leaving them ineligible for benefits once they begin looking for work again. Or because they worked part time and made the minimum wage or slightly above, leaving them short of the income requirements in some states. Or because, like many parents of children, they worked part time before being laid off and are again seeking only part- time work.

"There is a big discrimination against part-time people," said Richard B. Freeman, a labor economist at Harvard University. Like full-time employees, part-time workers pay unemployment-insurance tax while they are working, but in most states they do not receive the benefits of that tax when they are unemployed.

"It's not right," Mr. Freeman said.

Other people, meanwhile, never receive benefits because they do not think it is worth applying or because they do not know they qualify, labor experts said.

A report submitted to Congress last year by the General Accounting Office attributed the long-term decline in unemployment insurance coverage to several economic and political reasons. In past decades, more Americans worked in factories, where regular layoffs made jobless benefits a common part of their lives, and belonged to labor unions, which often reminded members to apply for benefits.

In the 1990's, more states eased eligibility rules than tightened them, Mr. Wenger said, helping the percentage of jobless workers getting benefits to increase slightly.

The General Accounting Office report recommended a more significant loosening of the rules so that unemployment insurance could catch up to the changes in the work force. The report noted, for instance, that over the last five decades many states have begun rejecting people who had quit their jobs for personal reasons even as the percentage of families with two working parents rose sharply.

The report also said unemployment insurance had become particularly important because of the number of low-wage workers who have entered the work force since the welfare system was overhauled in 1996.

But proposals to expand unemployment insurance have won little political support because they cost money and could give people less incentive to find a job, economists say. In fact, Mr. Freeman said, Australia and many European countries, while still more generous, have reduced their benefits in recent years, making their systems more like that of the United States.

This month, President Bush proposed extending benefits by 13 weeks for people who became unemployed after Sept. 11, who live in states where the unemployment rate rises by at least 30 percent and whose benefits expire. Most states normally allow people to collect for as long as 26 weeks.

The president also asked Congress to give states $3 billion in emergency funds, some of which could be paid to unemployed workers enrolled in job- training classes who are not normally eligible.

Still, with the country entering its first recession since 1991, economists said they were worried that the system's gaps could contribute to a delay in recovery.

"If you want to put money into the economy, put it in the hands of people who are the most desperate," said Richard J. DeKaser, the chief economist at the National City Corporation (news/quote), a bank based in Cleveland. "People who are unemployed are not going to save money."

If anything, many without jobs and unemployment benefits find themselves dipping into their savings or cutting back on their spending.

Marjorie Goldstein, who joined US Airways as a flight attendant in March after spending 15 years at home raising three children, lost her job late last month as part of the airline industry's huge cutbacks after the attacks. When she applied for unemployment, Pennsylvania rejected her and many of the people with whom she went to flight attendant school earlier this year, saying their work experience was too recent.

"If I were the major breadwinner, we'd be hungry," said Ms. Goldstein, who lives near Pittsburgh.

Because her husband works, she is not worried about paying the essential bills. But with her $1,700 in additional monthly income, the family had bought a new stove, refinished its driveway and paid for day camp for one of the children. Now, she said, they are likely to cut back.