The antigenic properties of the capsule polyoside (PRP) from Haemophilus influenzae type b (Hib) are adequate to form the basis for immunization designed to prevent the severe infections caused by this organism. The tolerance and immunogenicity of a vaccine containing either 12.5 or 25 micrograms PRP were studied in 325 healthy children aged 15 to 71 months after informed consent had been obtained from the parents. Each child was given one subcutaneous injection of 0.5 ml vaccine. Antibodies against Hib were assayed before and one month after the injection. Clinical tolerance was outstanding both locally (moderate and transient pain in 13% of cases) and systematically, with only eight (2.6%) febrile reactions in excess of 38.5 degrees C within 24 hours after the injection. A very significant rise in antibody titers was seen in all age groups, but a mean titer of 1 microgram/ml was achieved only in children aged 24 months or more. No significant difference was found between the two dosages. Individual analysis showed that following immunization antibody titers reached 0.15 micrograms/ml or more in 65% of infants aged 15 to 17 months, 71% of infants aged 18 to 23 months, 80% of infants aged 24 to 30 months and 95% of children older than 30 months. Despite the inadequate immune response evidenced in the younger age groups, our results confirm that Hib infections are preventable from the age of 2 years. Our results are consistent with those recorded with a similar vaccine in Finland.