Ninety-two patients and 28 staff members of an institute for mentally retarded patients were immunized intradermally with 2 microgram doses hepatitis B vaccine (HB-Vax), initially at month 0 and 1, and after evaluation followed by two booster injections at month 5 and 10, respectively. Blood samples were taken at months 2 and 11. Seroconversion rates of the two groups were similar on both occasions. Titre distributions for mentally retarded patients and staff members corrected for age, side effects and sex using multiple regression were significantly different at month 2 (p = 0.05) but not at month 11. Geometric mean titres for mentally retarded patients and staff members were 39 and 67 at month 2, respectively, and 1082 and 672 IU l-1 at month 11, respectively. Percentage anti-HBs/a(w) differed significantly between patients and staff members. Side effects, largely local (pigmentation), were observed with a frequency of about 35% in both mentally retarded patients and staff members. Since 92% of the mentally retarded patients had anti-HBs titres greater than 10 IU l-1 and approximately equal to 80% greater than 100 IU l-1, low dose vaccination, four injections by intradermal route, seems to be a suitable cost-reducing immunization system in institutes for mentally retarded patients.