U.S. Steel Corp., the nation's largest steelmaker, said yesterday that first-quarter earnings plunged to $79.9 million (88 cents a share) from $270 million ($3.06) for the same period a year ago.

U.S. Steel said its income reflected the performance of Marathon Oil Corp. U.S. Steel completed a $6.8 billion takeover of Marathon in March.

Meanwhile, second-ranked Bethlehem Steel Corp. reported a $66.7 million loss for the first quarter on $1.5 billion in sales. And LTV Corp., the parent firm of third-ranked J&L Steel Corp., said a sharp cut in steel shipments caused its first-quarter earnings to drop $27 million from 1981.

U.S. Steel said its earnings came on sales of $5 billion, up from about $3.4 billion for the 1981 first quarter. The 1981 first-quarter income included $84.7 million from the sale of coal reserves.

U.S. Steel, which supplies the automobile, housing and other industries, has been stunned by the nation's economic downturn.

U.S. Steel's quarterly steel shipments dropped from 4.2 million tons a year ago to 3.2 million tons, the lowest nonstrike level in more than 40 years, the corporation said.

Bethlehem, which earned $25.9 million (59 cents a share) on sales $1.8 billion, said its steel shipments declined 23 percent from a year ago to 2.3 million tons, the third straight quarterly drop.

All the steelmakers blamed the influx of foreign steel into the domestic market for low sales. Steel imports accounted for nearly 25 percent of market during the first two months of the year.

LTV, a Dallas-based conglomerate with interests in ocean shipping, aerospace, defense and energy products and services, said net income for the first quarter of 1982 was $20.3 million (39 cents a share), down from first-quarter 1981 earnings of $47.3 million ($1.30).

Revenues fell to $1.69 billion from $1.75 billion.

Eastern Airlines, which dominates the Northeast-to-Florida market, said it lost $51.4 million in the first quarter, largely because of the recession and fare wars.

In the 1981 first quarter, Eastern earned $4.1 million but showed a 4-cent-a-share loss after paying preferred stock dividends.

Operating revenues slipped to $908.4 million from $961.5 million in the 1981 quarter.

The carrier reported a first-quarter operating loss of $21.5 million compared with a $13.9 million operating profit a year earlier.

The 1982 results included a $2 million profit on the sale of equipment, down from $5.2 million a year earlier.

Xerox Corp. yesterday reported a 23 percent decline in first-quarter profits to $109.5 million ($1.29 a share) from $142.7 million ($1.69) a year ago.

Operating revenues were flat at $2.051 billion against $2.046 billion a year earlier. Chairman C. Peter McColough said sales would have been up about 6 percent except for the strength of the dollar abroad.

Coca-Cola Co. said yesterday that its first-quarter income from continuing operations increased by 4.7 percent to $102.2 million from the previous year.

Chairman Roberto C. Goizueta said income per share was 83 cents, or a 5.1 percent increase from 1981 first-quarter earnings.

Goizueta said the company's foreign soft-drink shipments increased approximate 10 percent during the first quarter, while retail sales of company soft-drink products in the United States increased an estimated 4 percent.

Full net for the quarter was the same as the continuing net but rose $2.4 million (2 cents a share) for the year-ago period because of discontinued operations of two firms. Sales for the quarter were $1.27 billion against $1.34 billion a year earlier, a 5.6 percent drop.

American Broadcasting Cos. said yesterday that it earned $24.17 million (84 cents a share) on revenues of $612.91 million in the first quarter,up from $12.76 million (45 cents) on revenues of $541.23 million a year earlier.

However, Chairman Leonard Goldenson said 36 cents a share of the profit resulted from tax credits on motion picture and television films produced in prior years.

Goldenson said ABC's television network had record first-quarter results from network programming and from the operation of network-owned stations.