The objective of this prospective clinical study was to evaluate the safety, tolerability and immunogenicity of Chiron Hib vaccine (Vaxem Hib) in Thai infants. This trial was conducted at Bhumibol Adulyadej Hospital, Bangkok, Thailand from June to November 1999. Three intramuscular injections of the vaccine were given to 119 infants at 2, 4 and 6 months of age. Reactions and adverse events after the vaccination were recorded. Blood samples for anti-PRP antibody were collected before the first immunization, and after the second and third immunizations. After the second dose, 91% and 58% of the subjects had anti-PRP antibody titers of > or =0.15 microg/ml and > or =1.0 microg/ml, respectively. After the third dose, 99% and 90% of the subjects had anti-PRP antibody titer > or =0.15 mcirog/ ml and > or =1.0 microg/ml, respectively. Local and systemic reactions were mild and transient. The study indicates that Vaxem Hib vaccine is safe and well tolerated. Three doses of the vaccine are necessary to achieve adequate protection in infants.