The stock market chalked up a moderate gain today, aided by some upbeat economic news.

Airline stocks climbed and energy issues fell on signs of new downward pressure on oil prices.

The Dow Jones average of 30 industrials rose 1.35 points to 1,347.45, closing out July with a net advance of 11.99 points.

Volume on the New York Stock Exchange reached 124.17 million shares, up from 102.27 Tuesday.

Before the market opened, the government reported that the index of leading economic indicators rose 1 percent in June. It was the largest increase in five months for the index, which is designed to detect future economic trends.

The index reading for May, which had previously been reported as an increase of 0.7 percent, was revised downward to plus 0.1 percent. But analysts said the leading indicators still seemed to be pointing toward reasonable growth in business activity in the months ahead.

Separately, the Commerce Department said new factory orders rose 1.9 percent in June.

Analysts said the statistics generally provided support for hopes that economic conditions would be healthy enough through the second half of the year to allow for an improvement in corporate profits.

The belief has spread on Wall Street lately that economic and earnings news holds the key to the stock market outlook, now that interest rates apparently have ended their long early-1985 decline.

In the airline sector, UAL gained 1 7/8 to 55; AMR 1 1/2 to 48 1/4; Delta Air Lines 3/4 to 49 1/8; Eastern Airlines 1 to 10 5/8, and Pan American World Airways 1/2 to 7 1/8. Pan Am led the active list on turnover of more than 4.1 million shares.

Among energy issues, Amoco fell 1 1/8 to 63 7/8; Chevron 1 1/8 to 36 1/4; Mobil 3/4 to 29 5/8; Texaco 7/8 to 36; Schlumberger 5/8 to 38 3/4, and Halliburton 1/4 to 29 7/8.

Middle South Utilities was the NYSE's second most-active stock, tumbling 2 1/8 to 11 7/8. The company said state regulators denied its Louisiana Power & Light Co. subsidiary a rate increase in connection with two nuclear power plants.

Levi Strauss climbed 1 3/8 to 49 1/2 in heavy trading. Late Tuesday, the company's directors approved plans for Robert Haas, its president, and members of his family to acquire the company for $50 a share in cash.

U.S. Steel, which announced a dividend increase on Tuesday, added 1/2 to 31.

Commonwealth Edison was off 3/8 to 30, Consolidated Edison was up 3/8 to 36 and Boston Edison was up 5/8 to 37 1/2. Southern Company was up 3/8 to 20 3/4 after reporting higher six-month earnings.

Bankamerica fell 5/8 to 16. The nation's second-largest bank said Tuesday it would eliminate 2,000 jobs in its world banking division over the next 12 to 18 months.

Colgate-Palmolive fell 3/4 to 27 7/8 after saying it foresaw a profit of $260 million on the sale of certain non-core businesses and that it would begin a stock buyback program.

Union Carbide lost 3/4 to 52. Massey Ferguson edged up 1/8 to 2 1/2. TRW eased 1/4 to 77 1/8. Dow Chemical rose 3/8 to 36 7/8.

In the daily tally on the Big Board, more than three issues rose in price for every two that declined. The exchange's composite index added .59 to 110.47.