The institutionalized population and staff in a centre for the mentally retarded were tested retrospectively, and 1.6% hepatitis Bs antigen and 0.6% hepatitis Be antigen carriers were found among persons with mental retardation. The 40 subjects, residents and staff, with most risk of acquiring hepatitis B were given vaccination. The immunogenicity of the H-B-Vax vaccine was followed up prospectively and, in 7.5% of the vaccinated persons, even re-vaccination gave an unsatisfactory result.