This study is designed to examine and compare seroconversion in response to IPV given according to the WHO/EPI and US schedules. Our primary interest is to determine whether the seroconversion rate among children receiving the WHO/EPI schedule is no lower than 10% less than the rate among those using the US schedule. To determine the required number of study participants, we will assume that the seroconversion rate using the US schedule will be 85% and we want power of .80 to find a significant difference between the rates if the rate among the WHO/EPI schedule group is 75% or lower. Using a one-tailed test (and realizing that one-sided significance levels are uncommon in epidemiologic studies and give us a confidence level of only 90% and not the standard 95%) based on the normal approximation of the binomial distribution, a sample size of ~220 subjects per arm will satisfy these criteria. To compensate for expected attrition of ~14 % , a goal of 250 infants will be set for each arm.