A comparison was made of the immunity conferred by injected formalin-killed poliomyelitis vaccine and orally administered attenuated poliomyelitis vaccine in Nigerian infants under tropical conditions where interfering enteroviruses caused poor conversion rates with attenuated poliomyelitis vaccine. The immunization schedules was completed on 230 infants. The levels of antibodies to polioviruses were assessed before immunization and at periodic intervals during the trial. Of the children tested, 74%, 72% and 85% lacked antibodies to poliovirus types 1, 2 and 3, respectively, at 6 mo. if they received no vaccine; the comparative proportions, respectively, were 52%, 8% and 48% if they had oral attenuated poliovaccine and 2%, 4% and 0% if they had 3 doses of inactivated poliovaccine. Killed poliovaccine incorporated in a quadruple vaccine may have a place in developing countries like Nigeria in the control of diphtheria, tetanus, whooping cough and poliomyelitis.