Changes of titers of acquired hepatitis B surface (HBs) antibodies against HB vaccine were investigated by measuring them again after a long lapse of time. Ten micrograms of the recombinant HB vaccine was intramuscularly injected respectively to the staff of the authors' hospital three times. Four weeks after a full course of the vaccination 185 persons could acquire antibodies, whose titers were 2.0 or more in cut off index (C.I.) by radioimmunoassay (RIA). Of these members titers of antibodies of 48 subjects could be remeasured for the first time 38 months after the measurement of the titers in the 4th week after a full course of the vaccination. Four weeks after the last vaccination 7 persons were high responders whose titers of antibodies were 50 or more in C.I. 29 were medium responders with their titers from 49 to 10, and 12 were low responders having titers of less than 10. However, 38 months after the course the titers of those responders decreased so much that nobody remained highly responders, 22 were low responders, and the remaining 22 turned to be negative again with titers less than 2.0. Supposing that the titer of antibody enough to protect the infection of HB virus is 10 or more in C.I., 32 of 36 persons needed booster shots 38 months later because their titers dwindled down to less than 10.(ABSTRACT TRUNCATED AT 250 WORDS)