In 1978 it was suggested that hepatitis B (HB) vaccine should be used to prevent the early hepatitis B surface antigen (HBsAg) carrier state in children. Immunization was effected by 3 injections of HB vaccine at one-month intervals followed by a booster injection after one year. Children in a control group were immunized with DT-polio vaccine according to the same schedule. The anti-HBs response of the children to HB vaccination was studied in relation to their hepatitis B virus (HBV) serum markers prior to immunization. Of the seronegative children, 70.5% responded to immunization after 2 injections of 5- g doses of HB vaccine and 94% after the third injection. The efficacy of the vaccine was demonstrated by comparison of HB events after one year in 309 seronegative children immunized with HB vaccine and 252 seronegative children immunized with DT-polio vaccine, and after two years in 101 and 119 children, respectively. The incidence of the HBsAg carrier state was reduced by 80% in susceptible children. In order to eliminate the perinatal transmission occurring in newborns with HBsAg-positive mothers, a study of immunization at birth has been instituted. A total of 86 newborns responded to the vaccination as well as older children, irrespective of the HBV status of their mothers. After one year, the incidence of the HBsAg carrier state was reduced by 80%. In Africa, immunization teams have a limited amount of time to devote to each rural community. The immunogenic effect of 2 doses of HB vaccine given at an interval of 2 or 6 months has therefore been investigated. All were given a booster dose one year after the first injection of vaccine. No difference was observed in the seroconversion rate or in the anti-HBs titres as between the two protocols. These results demonstrate that 2 doses of 5 micrograms of HB vaccine are sufficient to obtain a high immunogenic effect in infants. In addition, an investigation was carried out on the immune response to HBsAg and tetanus toxoid antigen when administered simultaneously to children as HB vaccine and DT-polio vaccine. The immune response was at least equal to that observed after administration of these vaccines separately.