Bethesda Research Laboratories Inc., a Gaithersburg biotechnology firm that grew rapidly since its birth six years ago, permanently laid off more than one-third of its work force yesterday.

The layoff, which cost approximately 150 workers their jobs, was generally across the board, affecting the company's divisions and types of workers more or less in proportion to their numbers in the company--except for the company's most credentialed scientists, who were laid off in somewhat less than proportion, a spokeswoman said.

The company said the layoffs reflected a reassessment of the company's technical and business operations. That reassessment resulted in restructuring the business and laying off 36 percent of the work force.

"The thing we want to stress is that the level of the product and services provided are unchanged," said Michelle Hartz, director of public relations. She said that the privately held company would not disclose its most recent quarter's results, but she said that the layoffs were not in response to any particular downturn in earnings or other economic bad news.

"The decision to restructure has been a long and hard decision. It reflects financial considerations. It also reflects an evaluation of our strengths, of our business acumen, our scientists--what they're good at--and the market," she said. "We really looked at the whole thing."

The company said it will reduce, but keep intact, its research products division. That division is the company's oldest and its major income source. It produces more than 500 items and serves more than 2,000 laboratories and 30,000 scientists.

The company's instrumentation division--which has introduced a nucleic acid analyzer and a "gene machine" that reads DNA--also will be left basically intact.

The company's molecular diagnostic division will be retained but with reduced research expenditures, the firm said. The division was established a few years ago to evaluate entering the medical diagnostic market, but some markets were not as great as anticipated and some products did not work out, said Hartz.

The genetic division, which pursues genetic engineering to produce abundant and low-cost amino acids, which are the building blocks of proteins used in food additives and other uses, will focus on that research and other selective projects.

"We anticipate that the new structure will benefit both the company and its shareholders on a long-term basis," the company said, announcing the layoffs. It is the type of reorganization often needed by a company that grows rapidly, said Hartz.

The company, founded in 1976 with three employes, had grown to 410 workers before the layoffs and had sales of $10 million last year.