Antibody responses were estimated in 42 healthy high risk volunteers submitted to three monthly intradeltoid inoculations of a 5 micrograms HBs vaccine containing both subtypes adw and ayw. The presence and concentration of anti-HBs varied according to individual responses and the number of injections. One month after the 1st dose those responding numbered 19%, one month after the 2nd the number rose to 83,3%, and reached 100% one month after the 3rd dose. In a few cases antibody levels were low (less than or equal to 50 mIU/ml), while in most subjects they rose after the 2nd dose, reaching high titres. Concordance and comparability of anti-HBs concentration were achieved by three different measurements (S/N, RIA Units, mIU/ml). No important side effects were observed. The timing of booster doses is discussed.