Human hepatitis B virus vaccine was prepared using antigen produced by recombinant technology in yeast (Saccharomyces cerevisiae). The highly purified antigen had the correct amino acid sequence and assumed the appropriate conformational structure to present the immunologic determinants (epitopes) that are needed to stimulate an appropriate immune response. Yeast-derived vaccine, was safe and was equally immunogenic and protective against hepatitis B as plasma-derived vaccine, as demonstrated in tests carried out in animals and in human beings. The yeast-derived vaccine produced by the Merck Sharp & Dohme Research Laboratories was licensed for general use in the Federal Republic of Germany in May and in the United States of America on July 23, 1986. It represents the first licensed vaccine of any kind produced by recombinant technology, and establishes the precedent for new vaccines to be made using this methodology.