The immunogenecity of highly purified, formalin inactivated hepatitis B vaccine (Hepavax-B), containing 20 pg of HBsAg for an adult and 10 pg of HBsAg for a child, was studied in 76 healthy volunteers (30 subjects under 10 years of age, 46 subjects over 20 years of age) who were serologically negative for hepatitis B virus markers-HBsAg, anti HBs and anti HBc- and without clinical or laboratory proof of hepatitis as evidenced by elevated serum aminotransferase levels. At intervals of one month, three doses of vaccine were injected to each volunteer on the deltoid area intramuscularly. Hepatitis B virus markers were detected by radioimmunoassay and quantification of levels of anti-HBs by radioimmunoassay S/N ratio units. The results were as follows; 1) Anti-HBs developed in 60.5%, 85.5% and 100.0% at 1,2 and 3 months after vaccination respectively. Anti-HBs responses in the child group and in the adult group were 60.0% and 60.9% at 1 month, 93, 3% and 80.4% at 2 months and 100.0% both at 3 months after the first vaccination respectively. There were no statistically significant differences in the anti-HBs response between the two groups. 2) Anti-HBs responses over 10 S/N ratio units by radioimmunoassay were noted in 28.9% at 1 month, 63.2% at 2 months and 84.2% at 3 months after vaccination. At 3 months after vaccination, anti-HBs responses over 10 S/N ratio units occurred in 93.3% in the child group and in 78.3% in the adult group (p=0.07). 3) There were no cases of elevated serum aminotransferase activities or development of HBsAg or anti-HBc during the 3 months. 4) There were no side effects except for the transient problems. The results suggest that anti-HBs responses after three injections of hepatitis B vaccine at one month interval seem to be as excellent and safe as other schedules, and that the younger the age of the recipient, the more rapid and higher anti-HBs responses are expected.