Fifty eight children aged 5-12 years (mean 9.3 years) who had received three x 2 micrograms of Merck Sharp and Dohme (MSD) H-B-Vax intramuscularly at time 0, 1, 6 months were given a further 2 micrograms dose 12 months after dose 3 and tested for immune response two weeks later. Fifty seven of 58 children were positive for antibody to hepatitis B surface antigen (anti-HBs) when tested by radioimmunoassay. In 56 of the 57, the response was anamnestic in nature. The geometric mean titre of anti-HBs rose from 106 IU/L before, to 15,759 IU/L after the booster dose. The latter figure was greater than that obtained in 20 adults given three x 20 micrograms of the same vaccine and also greater than that reported in children given three x 10 micrograms of MSD H-B-Vax. This study demonstrates that 8 micrograms of H-B-Vax given as four doses is at least as immunogenic in children as 30 micrograms of the same vaccine given as recommended and that the low dose strategy is appropriate for this expensive vaccine.