In order to clarify the serum anti-HBs response and persistence, and prophylactic effects to hepatitis B (HB) vaccination, 43 human healthy young adult volunteers who were seronegative for 5 serological markers of hepatitis B virus (HBV) and received intramuscularly HB vaccine (Hepaccine- B) at 0, 1 and 2 months were followed up for 36 months after the first injection. The heat-inactivated HB vaccine was prepared by Cheil Sugar Co. And contained 3 ug of HBs Ag. Blood specimens to be examined for the 5 serological markers of HBV and the aminotransferase activity were drawn at 1, 2, 3, 4, 5, 6, 12, 17, 24 and 36 months after the first injection. The five serological markers of HBV were detected by radioimmunoassay and the aminotransferase activity was measured by automatic kinetic method. The results obtained were as follows: 1) Anti-HBs response to the HB vaccine was noted in 17 of 19 (89,3%) male vaccinated subjects and all of 19 (100%) female vaccinated subjects at 4 months after the first injection, and were higher in females than in males. 2) Anti-HB response tc HB vaccine were 94.7% at the 4 months after the first injection and gradually decreased thereafter to 92.1% at 6 months and 86.8% at 24 months, and increased again to 94.4% at 36 months. 3) Negative seroconversion rate of anti-HBs was 8.3%(3/36) during the period of 24 months after the first injection. 4) Mean anti-HBs titers reached at peak at 9 months after the first injection in male subjects and 6 months in female subjects, and were higher in females than in males. 5) Non responders were 5.3% (2/38), low responders 18.4,o (7/38) and good responder 76.3% (29/39) during the period of 24 months after the first injection. 6) Anti-HBs titers of 11 vaccinated subjects reached at peak during the period of 5-6 months after the first injection and gradually decreased during 24 months and increased again at 36 months. It suggqsted that this may be due to booster results by natural infection of HBV. 7) No infection of HBV were noted in all of 38 vaccinated subjects during 24 months after the first injection and in all of 11 vaccinated subjects who were during 36 months respectively. On the basis of this findings the immunogenecity, persistence and prophylactic effect of this Hepaccine-B seem to be excellent during the period of 36 months after the first vaccination.