The study was conducted to assess the immunogenicity of three doses of two recombinant hepatitis B virus (HBV) vaccines, administered simultaneously with a DT vaccine or one of three different pertussis vaccines combined with diphtheria and tetanus toxoids. The study population consisted of 1237 children selected from the cohort of 15,601 children enrolled in the Italian trial on pertussis vaccines. HBV vaccination was performed at 2, 4 and 12 months of age, with the first two doses concurrent with OPV and DTP vaccination. The DTP vaccines administered in the pertussis trial included one whole cell DTP, licensed in the USA, and two three-component acellular DTaPs, manufactured in Europe. Immunogenicity to HBV was evaluated on serum samples collected 9 months after the third dose of HBV vaccine. Antibodies against HBsAg were detected by ELISA and expressed in mlU/ml. In 13 children, the antibody response was below the protective level of 10 mlU/ml-1. No statistical difference was found among the various study groups with respect to the proportion of children showing protective response. Higher humoral response was observed in children receiving mixed HBV vaccines in each pertussis study groups.