Philip Morris Inc. reported yesterday that its first-quarter net earnings are up 25.6 percent from last year, citing sales gains in its cigarette, brewing and soft-drink operations.

Philip Morris said it earned $138 million ($1.11 a share) in the quarter compared with $109.9 million (88 cents) in the first quarter of 1979.

Consolidated operating revenues for the first quarter were $2.3 billion, up from $1.9 billion a year ago.

The company, the country's second-largest cigarette producer and second-largest brewer, said its Miller Brewing division recorded higher sales despite a slight drop in shipments from a year earlier.

Merrill Lynch & Co., the country's largest investment firm, yesterday reported a 78 percent jump in its first-quarter earnings to $35.1 million (97 cents a share) from $19.7 million (54 cents) in 1979.

Revenue rose in the same period to $694.3 million from $438.1 million in the first quarter of 1979.

Chairman Donald T. Regan and President Roger E. Birk said that despite "the turbulent markets," their firm concluded the first quarter of 1980 with shareholders' equality of more than $800 million and total assets reaching $11 billion, both records for the company.

Regan said commission revenues virtually doubled in the 1980 first quarter, reflecting the surge in equity volume. Insurance revenues continued to grow due to gains by the Family Life Insurance subsidiary and interest rates in early 1980, he added.

Shearson Loeb Rhoades, Inc., another New York brokerage firm, reported a 438 percent increase in earnings for the first three months of the year to $19.4 million ($3.10 a share) from $3.6 million (68 cents) the year before.

Sanford I. Weill, Shearson's chairman and chief executive, said the sharp gains reflect record volume levels on the securities exchanges.

Republic Steel Corp. reported net earnings fell to $20.6 million ($1.28 a share) from $43 million ($2.65) a year earlier. Sales were down to $995.6 million from $1 billion in the first quarter of 1979.

The company said increases in labor, material and energy costs during the first quarter far outpaced the company's price increases.

Wheeling-Pittsburgh Steel reported first-quarter earnings of $6.5 million ($1.45 a share), down from $9.2 million ($2.20) a year earlier. Sales were down to $309.2 million from $313.4 million a year ago.

Union Pacific Corp., the railroad and fuel combine, reported 1980 first-quarter net income climbed 21 percent to $102 million ($2.13 a share) from $84.1 million ($1.77) a year earlier.

Sales rose to $1.25 billion from $901.8 million for the same period a year earlier.

The Champlin Petroleum Division had the best profit gain, to $52.8 million from $45.3 million a year ago. Union Pacific Railroad showed gains in earnings to $35.3 million from $32.8 million in the 1979 first quarter. The Rocky Mountain Fuel Division more than doubled its earnings to $14.6 million from $6.2 million a year earlier.

Uniroyal Inc., the tire and chemical concern, yesterday reported an estimated loss of $12 million in the 1980 first quarter compared with a $6 million profit in the first quarter of 1979.

Estimated sales for the latest quarters were $579 million, down from $712 million a year ago, Uniroyal President Joseph P. Flannery said.

Citing slumping auto sales and high gasoline prices, Flannery said the company is unlikely to show a profit this year.

Flannery attributed most of the sales decline in Uniroyal's tire division to ending European tire sales in the last quarter.

General Telephone & Electronics Corp. yesterday reported its profits dropped 59 percent in the first quarter, largely the result of losses stemming from its consumer electronics business outside the United States.

GTE said it earned $56.9 million ($38 cents a share) in the quarter, down from $140.1 million (97 cents) in the same quarter of 1979. Revenues rose 17 percent from $2.26 billion to $2.63 billion.

Although an $85 million writeoff in connection with the consumer electronics business was the main source of the drop, GTE said its operating income before the writeoff and before currency translation effects dropped from $145.97 million in the 1979 period to $134.5 million this year.

Maytag Co., the appliance maker, reported net income was $10.8 million (81 cents per share) on sales of $97.9 million in the 1980 first quarter compared with $9.8 million (73 cents) a year earlier on sales of $86.8 million. c

Chairman Daniel J. Krumm said the first quarter results were a good start on what seemed likely to be a difficult year for the company.