Fifty of the nation's largest corporations, with combined profits of more than $56 billion, paid no net federal tax from 1981 to 1984, and 48 had enough extra tax breaks to receive refunds totaling almost $2.4 billion, according to a new study by a labor-funded organization.

The study, by Citizens for Tax Justice, found that among major corporations, those most often escaping tax liability include large defense contractors -- Boeing Co., General Dynamics, Northrop Corp., and others -- oil and gas companies, utilities, timber manufacturers, aerospace firms, airlines, telecommunications companies and banks.

Meanwhile, industries benefiting from fewer tax breaks, such as high-tech and service firms, faced significantly larger federal tax bills during that period.

The study appeared likely to lend momentum to the administration's effort to revise the federal tax code. It comes less than a month after a Treasury Department study found that 30,000 households with incomes of at least $250,000 in 1983 used tax shelters to reduce their tax rates to less than 5 percent. The statutory rate for taxpayers in that bracket is 50 percent.

Similarly, the new study showed that deductions, credits and exemptions favoring capital investment and lending institutions allowed the 275 companies surveyed to reduce their average tax rate from the corporate rate of 46 percent to16.6 percent last year.

"It's a public scandal when members of the Fortune 500 pay less in taxes than the people who wax their floors or type their letters," said Robert S. McIntyre, the author of the study.

The companies lowered their tax rates legally by using accelerated depreciation and the investment tax credit, both targeted to help capital-intensive industries. Defense contractors also avoid taxes under a provision that lets them defer tax bills on some income for decades.

Citizens for Tax Justice released a similar widely publicized study last year that prompted a public outcry over the "unfairness" of the tax code. Many congressmen said during this recess that constituents asked why companies such as General Electric, Boeing and others have lower tax rates than they do.

The new study incorporates tax data from 1984 with the earlier study covering 1981-83. Last year, 40 corporations surveyed paid no taxes or received rebates, the study reported. It said that AT&T received the largest refund: $241.6 million on profits of about $1.9 billion.

An AT&T spokesman said the company piled up large tax losses last year and used them against income from earlier years, but called this "unique" to 1984.

Over the four years, the study showed Boeing was the largest tax-avoider, receiving rebates of$285 million on about $2.1 billion in profits. A Boeing spokesman said the company had no comment. The study showed six companies other than Boeing with net tax rebates of more than $100 million during the period: Dow Chemical Co.,$180 million; ITT, $177 million; Tenneco, $166 million; Pepsico, $135.8 million; Santa Fe Southern Pacific Corp., $133.4 million, and General Dynamics, $103.8 million.

Daniel Lundy, ITT's senior vice president in charge of taxes, said that his company did not receive a refund, but rather used extra tax credits from recent years to cancel out deferred tax liabilities that were coming due from the 1970s.

The survey also showed nine large, profitable companies paying taxes of more than 40 percent, including two textile firms -- VF Corp., 44.7 percent, and INTERCO, 43.7 percent -- as well as Whirlpool, 42.4 percent, and Ralston Purina Co., 42.1 percent.