Martin Marietta Corp. of Bethesda yesterday reported a jump in 1985 net earnings of roughly 37 percent to $249.3 million ($4.36 per share) on revenue of $4.4 billion, compared on a fully diluted basis with earnings of $176 million ($3.02) on revenue of $3.9 billion last year.

Also yesterday, USAir Group said its 1985 earnings declined, while Legg Mason Inc. posted a substantial increase in profit.

The diversified aerospace and high-technology company said fourth-quarter net earnings rose about 50 percent -- to $50 million (89 cents) -- on revenue of $1.2 billion, up from net income of $33.2 million (57 cents) on revenue of $1.1 billion for the fourth quarter of 1984.

The full-year net earnings include $73 million from gains on the sale of the Master Builders Division in 1985 and $11 million from gains from asset sales in 1984.

Thomas G. Pownall, chairman and chief executive officer, said, "the year's results reflect excellent sales and earnings gains in aerospace and continued strong performance in construction aggregates," such as crushed stone, gravel and construction sand. The company owns a construction materials division.

*USAir Group yesterday reported 1985 net earnings of $109.8 million ($3.98) on operating revenue of $1.7 billion, down 7.2 percent from net earnings of $118.3 million ($4.46) on operating revenue of $1.6 billion last year.

The company, based in Washington, said fourth-quarter earnings were up 5.6 percent -- to $29.3 million ($1.07) -- on operating revenue of $430.6 million, from $27.8 million ($1.02) on revenue of $414.9 million, in the fourth quarter last year.

"We are pleased with our 1985 performance, which continues our record of profits in today's turbulent airline industry environment," said Edwin I. Colodny, USAir Group chairman and president.

*Legg Mason Inc. yesterday reported a 173 percent increase in earnings for the third quarter of 1985 ending Dec. 31.

The Baltimore holding company providing securities brokerage and investment banking services reported earnings of $2.3 million (48 cents per share) on revenue of $29.2 million in the third quarter ending Dec. 31, compared with earnings of $854,000 (18 cents) on revenue of $18.9 million for the same quarter last year. Revenue grew 54 percent over the third quarter of 1984.

For the nine months, earnings increased 244 percent to $5.2 million ($1.11) on revenue of $77.1 million, compared with earnings of $1.5 million (33 cents) on revenue of $49.8 million a year ago.

Chairman and Chief Executive Officer Raymond A. Mason said the stock market and daily number of shares traded "exploded" last November to the company's benefit.

"Commission revenue was up sharply" from brokerage services the company provides, he said.