A polyribosylribitol phosphate (polysaccharide)-tetanus protein conjugate vaccine (PRP-T) against Haemophilus influenzae type b (Hib) was evaluated for safety and efficacy after vaccination of more than 100,000 infants. No major side effects were attributed to the vaccine. Immunogenicity studies showed an antibody response in 70% to 100% of infants after two doses, and in 98% to 100% of infants after three doses, within the first 6 months of life. Antibodies persisted in 90% of recipients, in whom significant anamnestic responses developed after a booster dose at 18 months of age. In comparison with other available Hib vaccines, PRP-T induces equal or higher mean titers after three doses. Although licensure of other vaccines interrupted controlled efficacy trials, up to that point five cases of Hib disease in those trials had occurred in placebo recipients, and no Hib disease has been reported in the more than 100,000 vaccinated infants who have received more than one dose of PRP-T. Thus PRP-T combined immunogenicity early in life with induction of immunologic memory.