Some of the greatest beneficiaries of the revolutionary advances in biotechnology over the past 15 years have been producers of diagnostic reagents, especially for the cloning and expression of antigens, primarily of viral origin. Recombinant DNA technology provides methods for producing antigens for diagnostic assays that are more highly purified, more specific, and safer to produce than viral culture and that are significantly less expensive to manufacture. Antigens so produced can be used for production of antibodies or antisera for competition assays, as reagents for mapping epitopes, as affinity-chromatography ligands for purification of antibodies or protein, and as research reagents. Their initial use in some hepatitis B assays may be primarily a cost-reduction application, but in other applications (e.g., HIV diagnostic tests) they present the first opportunity to commercially produce an otherwise very expensive antigen. Recombinant-DNA-produced antigens are also being used to develop safer vaccines, but not, however, without some consideration of the structural nature of immunodominant epitopes and the adequacy of the immune response.