Recombinant deoxyribonucleic acid technology has permitted the development of a vaccine from the hepatitis B surface antigen expressed in genetically manipulated yeast cells. Clinical trials with the vaccine were started in February 1984 and, to date, have involved more than 12,500 persons. The vaccine has been shown to be safe, well tolerated, and immunogenic in healthy persons of all ages and in special target groups likely to require vaccination. The vaccine's protective efficacy has been established in three groups at high risk for hepatitis B infection--homosexual men, institutionalized mentally handicapped patients, and neonates of mothers who are chronic carriers. Production of this vaccine on a large scale should make it less expensive than plasma-derived vaccines and thus broaden the indications for vaccination.