The reactogenicity and immunogenicity of simultaneous administration of recombinant DNA hepatitis B vaccine with diphtheria and tetanus toxoids (DT) and oral poliovirus vaccine (OPV) in 111 infants were compared with those of DT and OPV alone in a control group of 21 infants. All subjects received three doses of the vaccine according to one of three different schedules of vaccination. Reactions following simultaneous administration of vaccines were all but absent, with mild pain reported for four out of 111 subjects, compared with one of 21 in the control group. Seroconversion rates of 98-100% and high anti-HBs geometric mean titres (GMTs) were observed in all study groups after three doses of hepatitis B vaccine. Significantly higher anti-HBs were seen in Group III, where six months is allowed between the second and the third hepatitis B vaccine doses, compared with Group I and II, where only 1-2 months separate the second and third doses. A fourth dose of vaccine was needed in both these groups to obtain anti-HBs levels as high as seen in Group III after three vaccine doses at 3, 4 and 10 months of age. The immune response to DT and OPV was similar in the study groups and the control group. It is concluded that a course of 10 micrograms doses of recombinant hepatitis B vaccine given simultaneously with DT and OPV elicits a strong anti-HBs response and does not interfere with the immune response to the other antigens.