A formalin-treated hepatitis B vaccine in the form of purified hepatitis B surface antigen (HBsAg) was prepared from asymptomatic human HBsAg carriers. Its safety and potency were tested in 5 chimpanzees. The vaccine was administered to 264 individuals. The results of the first 173 immunizations, 46 hemodialysis patients and 127 staff members, are presented. Potency was ascertained and efficacy assessed by the development of humoral immune responses to HBsAg (anti-HBs antibody) and by seroepidemiologic studies of vaccinated and nonvaccinated subjects. The results, 2 yr after immunization, suggest that the vaccine was protective against hepatitis B infection in high-risk hemodialysis settings. Preliminary studies with an inactivated hepatitis B vaccine similarly prepared, but with Al(OH)3 as adjuvant, indicate that such a preparation induces a more rapid and stronger anti-HBs response.