The hemagglutination-inhibition (HI) antibody responses of 452 children to HPV-77, DE5 rubella vaccine were studied. Results indicated that seroconversion rates and geometric mean titers in infants inoculated at 12 through 14 months of age are comparable with those achieved in older children. "Vaccine failures" seem to occur randomly in vaccinees after 1 year of age; the infectivity of individual vaccines is questioned. The persistence of HI antibody was evaluated through serial sampling up to three years postinoculation.