One-hundred and eighty Chinese children [age range 5 months to 12 years, seronegative for all hepatitis B virus (HBV) markers] of parents seropositive for HBV surface antigen (HBsAg) were randomized to receive doses of either 10 or 20 micrograms of recombinant yeast-derived HBV vaccine at intervals of 0, 1, and 6 months. Six children defaulted and three other children (1.7 per cent) seroconverted to anti-HBc positivity without detectable HBsAg in the serum. All other children attained an anti-HBs titre of > 10 mlU/ml after three doses. Both 10 and 20 micrograms/dose regime gave a similar geometric mean titre (GMT) of anti-HBs. Children aged 0-4 responded with a similar titre compared with children aged > 4. Female children responded with a significantly higher GMT than male children and this was due to a high proportion of female children with higher peak titres. No major side-effects were encountered. We conclude that recombinant HB vaccine is highly immunogenic, well tolerated and equally effective with doses of both 10 and 20 micrograms and that girls responded with a higher anti-HBs titre compared with boys.