An enzyme-linked immunosorbent assay (ELISA) has been developed and validated to quantitate IgG1 and IgG2 antibody to polyribosyl-ribitol phosphate (PRP), the capsular polysaccharide of Haemophilus influenzae type b (Hib). The sera of children and infant Rhesus monkeys immunized with an Hib conjugate vaccine composed of Hib PRP covalently linked to an outer membrane protein complex (OMPC) from Neisseria meningitidis serogroup B (PedvaxHIB, PRP-OMPC, Merck, Sharp and Dohme Research Laboratories). The solid-phase antigen employed in the ELISA is a conjugate of PRP to human serum albumin. The enzyme-labeled antibody is alkaline phosphatase-conjugated mouse monoclonal (mAb) anti-human IgG1 or IgG2. A human serum standard was calibrated using parallel titrations with a known antibody standard. The geometric mean titer (GMT) of the anti-PRP IgG1 response to one dose of PedvaxHIB was 3.87 micrograms/ml (n = 82), 11.80 micrograms/ml (n = 62) and 14.57 micrograms/ml (n = 74) in infants and children 12 to 17 months, 18 to 23 months and greater than or equal to 24 months old, respectively. Infants 2 to 11 months old responded with an IgG1 anti-PRP response of 7.10 micrograms/ml while infant monkeys responded with a GMT of 150.65 (n = 9) after two doses of vaccine. The anti-PRP IgG2 GMT responses in all groups were less than 0.25 micrograms/ml, except for humans greater than or equal to 18-months old who exhibited a GMT of greater than or equal to 0.40 micrograms/ml (n = 75). PedvaxHIB, immunization of human infants and children and infant Rhesus monkeys elicits primarily an IgG1 response to PRP. The monkey model appears to be a reliable indicator of the human immune response.