Seventy-six children (aged 17-19 months) received 10 micrograms of Haemophilus influenzae type b polyribosyl-ribitol phosphate (PRP) vaccine, diluted with either phosphate-buffered saline (PBS) or diphtheria-tetanus-pertussis (DTP) vaccine, in a single-blind randomized trial. There were few side effects when PRP was administered alone. Before vaccination 37 of 76 children (49%) had non-protective antibody levels (less than 0.15 micrograms/mL); 26 of these 37 (70%) achieved antibody levels of greater than 0.15 micrograms/mL 1 month after vaccination. Before vaccination 16 of 76 (21%) had antibody levels of greater than 1.0 micrograms/mL; 1 month after vaccination 39 of 76 children (51%) achieved levels of greater than 1.0 micrograms/mL. Of 12 infants who had antibody levels less than 0.15 micrograms/mL 1 month after immunization, 10 had protective levels 18 months later. Administration of PRP mixed with DTP did not affect antibody response to PRP. The potential use and limitations of PRP vaccine are discussed.