The nation's two largest steelmakers reported yesterday that they lost money in the first quarter.

U.S. Steel Corp., which is No. 1, said that it lost $118 million in the latest quarter compared with net income of $80 million in the first quarter of 1982, while No. 2, Bethlehem Steel Corp., said it lost $175.2 million compared with a $66.7 million loss a year earlier.

Among other reports yesterday:

* Eastern Airlines lost $60.7 million compared with a $51.4 million loss for the same period last year, mainly because of "unprofitable deep-discount fares, which Eastern was forced to match during most of the quarter," and higher labor costs, Chairman Frank Borman said.

* American Broadcasting Cos. Inc.'s profit before one-time gains was unchanged from a year earlier on an 8 percent increase in sales.

* Standard Oil Co. of California's first-quarter profit jumped 34.8 percent.

Income at U.S. Steel's Marathon Oil Co. unit dropped sharply in the first three months of the year, while the steel segment and other businesses operated in the red, the company said.

First-quarter sales totaled $3.9 billion, down from $4.3 billion in the fourth 1982 quarter and $5 billion in the first 1982 quarter.

Steel operations alone lost $232 million in the first three months of this year, far above the $24 million loss a year ago.

But U.S. Steel Chairman David Roderick said operating results showed a "substantial improvement" from late 1982, reflecting savings from the new labor contract with the United Steelworkers union, a modest pickup in shipments and higher operating rates.

Bethlehem Steel, which suffered the largest quarterly loss in U.S. corporate history at the end of 1982, said that sales fell sharply, from $1.564 billion to $1.119 billion.

Bethlehem's operating loss also rose, to $163.2 million from $111.6 million.

Increased orders gave Bethlehem its first quarterly improvement in shipment levels in almost two years, to about 2 million tons.

Eastern Airlines report of the latest loss follows the company's recent disclosure that it will be forced into default by June if it can't restructure its debt.

Eastern said its operating revenue rose to $969.7 million in the quarter, up from $908.4 million.

American Broadcasting Cos. Inc. said its first-quarter profit both this year and in 1982 was $13.9 million (48 cents a share).

A year ago, ABC also had a $10.3 million extraordinary gain from an investment tax credit refund and other interest income, lifting net income to $24.2 million (84 cents). Thus, on a net-income basis, ABC's latest profit was down 42 1/2 percent from a year earlier.

First-quarter sales rose to $663.1 million from $612.9 million.

ABC said pre-tax earnings of its broadcasting group, which includes the ABC Television Network, rose 38 percent to $40.6 million from $29.3 million. Broadcasting revenue climbed 12 percent to $588.2 million from $526.6 million.

But ABC said its Video Enterprises group, which includes DAYTIME, Satellite News Channel and other cable-TV ventures that have required heavy start-up costs, posted a pretax loss of $11.6 million compared with a year-earlier loss of $2.17 million.

Standard Oil Co. of California, the fourth-largest U.S. oil producer, said that its first-quarter profit rose to $310 million (91 cents a share) from $230 million (67 cents) in the same period last year. Revenue fell to $6.99 billion from $10.02 billion.

Chairman George Keller said his company's gain was due to reduced exploration expense and a drop in the purchase price of Saudi Arabian crude oil.