This study was designed to assess the immunogenicity of a vaccine combining diphtheria and tetanus toxoids, acellular pertussis vaccine, and inactivated poliovirus vaccine reconstituting Haemophilus influenzae type b polysaccharide conjugated to tetanus protein (DTaP-IPV//PRP-T; Pasteur Merieux Connaught, Lyon, France) administered simultaneously in association with hepatitis B vaccine (RECOMBIVAX(R) Merck, Sharp & Dohme, West Point, PA, USA) for the primary immunization of infants. The vaccines were administered at two, three and four months of age. One hundred and sixty-two healthy infants, aged 8-10 weeks, were enrolled in the study. Blood samples were taken before the first dose and 4 weeks after the third dose. The infants were observed for 15 minutes after vaccination for any immediate reaction. Adverse events requiring a medical consultation were recorded by the parents in a diary over the 7 days following vaccination. Four weeks after the third immunization, the percentages of infants fulfilling seroconversion criteria were 98.9% for pertussis toxin, 95.9% for filamentous haemagglutinin, 100.0% for tetanus, 100.0% for diphtheria, 99.3% for poliovirus type 1, 100.0% for both poliovirus types 2 and 3, 98.0% for Haemophilus influenzae type b, and 100% for hepatitis B surface antigen. No vaccine-related serious adverse event was reported. The simultaneous administration of DTaP-IPV//PRP-T and hepatitis B vaccines at two, three and four months of age yielded clinically satisfactory immune responses to all antigens compared with historical controls and gave a good safety profile. (C) 1999 Published by Elsevier Science Ltd. All rights reserved.