The Haemophilus influenzae capsular polysaccharide-outer membrane protein conjugate, PRP-OMPC (PedvaxHIB) elicits very good antibody responses in infants > or = 2 months of age after the first dose. Increasing age at time of first vaccination correlates with higher antibody responses. Anti-PRP responses are consistently high with the first injection among all population groups studied. Booster doses stimulate anamnestic antibody responses after one year of age. Among US children (excluding Navajo and Apache children) given a primary injection at 14-18 months of age, the geometric mean titre (GMT) after 2 to 3 years was > 1 micrograms/ml. US children (excluding Navajo and Apache children) given a primary series at 2 and 4 months of age and a booster at 18 months of age also had an anti-PRP GMT > 1 micrograms/ml 2.5 years later. Navajo and Apache children given a primary series at 2 and 4 months of age and a booster at 12-15 months had antibody levels of 1.50 micrograms/ml one year later. Antibody persistence data suggest there will be long-term protection against Haemophilus influenzae b disease following immunization with PRP-OMPC.