Suffering from a slump in demand for steel and higher operating costs, Bethlehem Steel Corp. today reported a loss of $39.2 million, or 60 cents a share, on sales of $1.05 billion in the first quarter.

The loss compares with a profit of $21 million, or 20 cents a share, on sales of $1.2 billion in the first quarter of 1990.

Including payments to holders of preferred stock, the net loss was $45.3 million, compared with a profit of $15 million a year ago.

The company said it would issue a dividend announcement tomorrow. Bethlehem Steel has paid 10 cents a share for the last six quarters.

Steel operations, which include the Sparrows Point plant in Baltimore County, reported a loss of $26.3 million compared with income of $28.6 million in the first quarter of 1990, as the result of lower prices and a 7 percent decline in steel shipments.

At the same time, labor and iron ore costs increased.

Chairman Walter F. Williams said raw steel production was 67 percent of capability compared with 70 percent a year ago.

The 1991 rate would have been even lower except for the need to build inventories for a blast furnace renovation at the Burns Harbor, Ind., plant and a planned 40-day outage of the Sparrows Point hot strip mill, which is being modernized.

The mill is to be closed in May and not resume full operation until July.