The objective of this study was to evaluate the immunogenicity and safety of a pentavalent vaccine (Pentaxim) containing diphtheria, tetanus, acellular pertussis, inactivated poliovirus, and Hib polysaccharide-conjugate (DTaP-IPV//PRP-T) antigens, in Thai children. One hundred eighty-six infants who had received a hepatitis B vaccine at birth were given a pentavalent vaccine at 2, 4 and 6 months of age and a hepatitis B vaccine concomitantly at 2 and 6 months of age. Immunogenicity was high for each vaccine antigen. The study vaccine was well tolerated and side effects were few. After the third dose, 100% of subjects had an anti-PRP > or = 0.15 microg/ml and 96.5% > or = 1.0 microg/ml; the anti-PRP GMT was 9.53 microg/ml. Seroprotective rates for diphtheria and tetanus (> or = 0.01 IU/ml) were 99.4% and 100%, respectively, and 100% for all three poliovirus types (> or = 8 1/dil U). The vaccine response rates to pertussis antigens (a 4-fold increase in antibody titer were 94.1% for PT and 93.0% for FHA. The DTaP-IPV//PRP-T vaccine given at 2, 4 and 6 months of age concomitantly with a monovalent hepatitis B vaccine, was well tolerated and highly immunogenic for primary immunization of infants in Thailand.