Life Technologies Corp. of Gaithersburg yesterday reported an 11 percent rise in its third-quarter earnings, citing higher sales of products that biotechnology companies use in research and manufacturing.

Sales of its molecular diagnostics tests were essentially flat from year to year, while Life Technologies gained $2.3 million more revenue from favorable currency rate changes in the third quarter than it did in the same period a year earlier, the company said.

Life Technologies earned $3.6 million (25 cents a share) in the third quarter, compared with a profit of $3.2 million (23 cents) in the 1989 third quarter. Its revenue rose 19 percent, to $38.2 million from $32.1 million in the year-ago quarter.

In the first nine months, the company reported a profit of $10.5 million (74 cents), up 6 percent from earnings of $9.9 million (70 cents) in the same period a year earlier.

Revenue increased 10 percent, to $111.9 million from $101.6 million.

Williams Industries Inc., the Falls Church-based construction company, said its profit more than quadrupled in the fiscal year ended July 31 as it worked out problems with a Richmond subsidiary and related companies.

For the fiscal fourth quarter, Williams's profit fell more than 8 percent.

The company said it earned $1.45 million in the fiscal year, compared with $349,574 in the previous fiscal year. Per-share earnings rose to 60 cents from 15 cents, and revenue gained nearly 17 percent, to $118.8 million from $101.8 million.

Williams said its fourth-quarter earnings totaled $533,346 (22 cents a share), compared with $582,980 (25 cents) in the 1989 fourth quarter. Revenue rose almost 2 percent in the quarter, to $26.9 million from $26.4 million.

A company spokesman said Williams's Richmond-based Creative Iron Inc. subsidiary reversed a loss in the fiscal year. Last year's results also reflected delays in opening a new concrete block plant in Richmond that forced the company to keep an obsolete, unprofitable plant open longer than planned to keep customers supplied.

The soft commercial building market has taken a bite out of Williams's backlog of work on hand, the spokesman said, but the backlog remains strong because of the company's role in bridge and industrial construction. Williams predicted higher profit in fiscal 1991, despite an expected decline in revenue.

Ethyl Corp. reported modest gains in earnings for the third quarter and first nine months. The Richmond-based chemicals producer said its profit totaled $56.2 million (47 cents) in the third quarter, 3 percent higher than $54.8 million in the 1989 third quarter.