A Gaithersburg medical research and technology company announced yesterday what was described as the first successful application of modern research techniques to diagnose an infectious disease.

Bethesda Research Laboratories Inc. -- a firm founded here in 1976 that now has an annual volume of more than $5 million in exotic enzymes and biomedical products -- said its scientists used recombinant DNA technology in diagnosing hepatitis-B, commonly called serum hepatitis, which results in destruction of liver cells.

DNA is deoxyribonucleic acid, which is localized in the nucleus of a cell and is the molecular basis of heredity in many organisms. Recombinant technology is a process of attempting genetic of DNA combinations in the laboratory.

In the development announced yesterday, Bethesda Research said its technology can detect the DNA of the hepatitis virus present in a sample of serum obtained from a patient.

According to Mark Berninger, a scientist who developed the process, applications from this breakthrough would include testing the safety of blood products and hepatitis vaccines and monitoring antivirus therapies.

Hepatitis-B is a major world health problem, with some 200 million chronic 700,000 persons in the United States, according to Bethesda Research. Following infection, many individuals become carriers and spread it to others.

Stephen Turner, the founder and president of Bethesda Research, said the hepatitis diagnosis process could be a prototype for other applications of recombinant DNA to disease discovery.

Noting that much discussion about commercial application of DNA research has centered on production of such hormones as insulin (used to treat and control diabetes), Turner emphasized: "Brl's application . . . potentially opens a new area of industrial products which could reach the marketplace sooner than other recombinant DNA research