Conjugate vaccines against Haemophilus influenzae type b (Rib) may modify Rib pharyngeal colonization. Rib colonization was compared in 371 infants and their families. In Oxfordshire, infants received PRP-T (polyribosylribitol phosphate conjugated to tetanus toxoid) and in Buckinghamshire they did not (controls). Infants were followed at 6, 9, and 12 months of age. Also, 6 unvaccinated Rib carriers were vaccinated and followed for 6 weeks. Rib acquisition was lower in vaccinees than controls (P < .01). During surveillance, 1.5% of vaccinees and 6.3% of controls carried Rib (P = .04). Among those with family Rib exposure, the carriage rates were 8.7% and 38.5% (P = .07), respectively. Rib carriage rates were lower among vaccinees' unvaccinated siblings. Giving conjugate vaccine to a child carrying Rib did not rapidly terminate carriage. Thus, a primary means by which herd immunity to Hib is induced in a vaccinated population may be through reduction or delay in the initial acquisition of Rib.