Production of polypeptide and protein antigens through recombinant DNA technology in prokaryotic and certain eukaryotic cells in culture is facilitating the development of new vaccines that are safe, efficacious, and economically feasible to manufacture. A current example is that of human hepatitis B vaccine that, to the present, has been produced commercially using hepatitis B viral surface antigen (HBsAg) purified from the plasma of human carriers chronically infected with the virus. Production of plasma-derived vaccine is limited by the available supply of suitable carrier plasma and by the need to apply highly technical procedures to purify the antigen as well as to ensure inactivation of all infectious agents that might be present in human plasma.