Two groups of infants received poliomyelitis immunizations beginning at the age of 6 weeks and completing the series by the age of 10 months. One group was given the vaccine as a separate injection while the other received the vaccine mixed in the same syringe with pertussis or diphtheriapertussis-tetanus vaccine. The responses of these infants to poliomyelitis vaccine were analyzed and the following general conclusions were reached: Poliomyelitis vaccine administered to infants as young as 6 weeks, and given according to the usually recommended schedule results in a satisfactory antibody response. The response to poliomyelitis vaccine mixed with pertussis and DPT vaccines and given as one injection is satisfactory. The combination of antigens gave no practical interference to development of antibodies to diphtheria and pertussis. There were no apparent hazards or adverse reactions associated with combined poliomyelitis-pertussis or poliomyelitis-DPT vaccines. There were no differences in poliomyelitis antibody responses following immunizations with separate and combined vaccine injection. The existence of maternal antibodies at the time of poliomyelitis immunization had no apparent influence on the production or persistence of active antibodies. These findings indicate that poliomyelitis immunization procedures can very easily be integrated into routine pediatric immunization programs, thereby affording the earliest possible protection.