Two hundred and sixty-seven susceptible hospital personnel who had been randomly divided into three groups to receive different doses (5 micrograms, 2 micrograms or 1 microgram) of a plasma-derived hepatitis B vaccine on a four-dose schedule were investigated annually for 4 years. Of them, 251 were vaccine responders. The percentages of persistence of antibody to hepatitis B surface antigen (anti-HBs) in these 3 groups were 95.5% (84/88), 92.3% (72/78) and 95.3% (81/85), respectively (P greater than 0.05). During the follow-up period, 12 of 21 (57.1%) responders with low anti-HBs titres (10-100 miu/mL) and 3 of 48 (6.3%) responders with medium anti-HBs titres (101-1000 miu/mL) were found to be anti-HBs seronegative, while none of the 182 candidates with high anti-HBs levels (greater than 1000 miu/mL) lost their anti-HBs. The 4 year cumulative rate of natural booster in the responders was 11.6% (29/251). None of the candidates became HBsAg positive during the follow-up period. This study revealed that low dose hepatitis B vaccine can provide satisfactory immunogenic response and long-term efficacy in Chinese adults and that the persistence of immunogenicity is not related to the vaccine dose but to the candidate's own initial anti-HBs response.