The efficacy of the hepatitis B vaccine Hevac B Pasteur was assessed in 277 persons over a period of 6 months. HBs antigen of 5 micrograms/dose was administered three times in monthly intervals. Before each vaccination and after 6 months 18 chemical pathology data and a clinical assessment were performed and all important serologic hepatitis markers (anti-HBs, anti-HBc, HBs antigen, HBc antigen, anti-HBe) were determined radioimmunologically. Within two months antibodies against the surface antigen (anti-HBs) had developed in 85.7% of vaccinees. After 6 months 93.1% of vaccines had become seropositive. Within the first two months anti-HBs-positive findings and higher anti-HBs concentrations occurred significantly more frequently (P less than 0.048) in females than in males. These differences had largely disappeared at the follow-up assessment in the sixth month. In contrast, both parameters (seroconversion rate and anti-HBs concentration) were age-dependent. Among vaccines of less than 25 years of age and of more than 35 there were significant differences after 6 months (P less than 0.005). The hepatitis B vaccine of the Pasteur institute can be considered immunogenic and safe.