An inactivated poliovirus vaccine prepared from cultures of Vero cells has been tested on 61 infants from two to 11 months of age (mean age, 4.3 months) for tolerance and serologic potency. Three doses of vaccine were given one month apart at the same time that diphtheria-tetanus-pertussis vaccine was injected at another body site. Poliovirus-neutralizing antibody titers were measured before the first and third injections and one month after each. The titer was considered positive when the dilution was greater than or equal to 1:4. The tolerance has been good. Thirty-one infants were assessable for serologic efficacy; all had significant serologic responses after two injections of polio vaccine, regardless of the titer of maternally transmitted antibodies before the immunizations. The third injection did not significantly increase the antibody titer observed after the first two doses.