• The conventional formulation of injectable poliovirus vaccine (inactivated) contains 20, 2, and 4 D-antigen units of types 1, 2, and 3 polioviruses. Primary immunization requires three doses given at intervals of at least four weeks. A new formulation with 40, 8, and 32 D-antigen units of the three poliovirus types has been prepared to reduce primary immunization to two doses. We evaluated the immunogenic efficacy of this new formulation supplied to us as a liquid vaccine containing diphtheria-pertussis-tetanus vaccines and inactivated poliovirus vaccine. Two doses were administered four weeks apart to 100 infants and eight weeks apart to 114 infants. Antibody titers were determined against the three types of polioviruses before and after immunization. The effects of age, presence of maternal antibody, and interval between doses on the frequency and titers of antibody response were assessed. Irrespective of age or interval between doses, the seroconversion rates to types 1 and 3 antigens were 90% to 100%. To type 2 antigen the rate was below 84% in the 6- to 7-week-old infants, 88% to 95% in 8- to 12-week-old infants, and 90% to 100% in 13- to 45-week-old infants. The seroconversion rates and geometric mean titers of antibody were lower in those infants with maternal antibody than in those without maternal antibody at the time of receiving the first dose. The best results were in infants 8 weeks of age or older, in whom the two doses were given eight weeks apart. We recommend this schedule.