John, T. J. and P. Jayabal (Enterovirus Lab., Christian Medical College and Hospital, Vellore, Tamil Nadu, India). Oral poliovaccination of children in the tropics. I. The poor seroconversion rates and the absence of viral interference. Am J Epidemiol 96: 263–269, 1972.—A study of the efficacy of trivalent oral polio vaccine (OPV) was conducted in a group of children between the ages of 3 months and 6 years, in Vellore, Tamil Nadu. Of the 191 children given the 1st dose of OPV, 72, 93 and 94 were found to be seronegative to poliovirus types 1, 2 and 3, respectively, prior to vaccination. The seroconversion rates in them were 35% to type 1, 76% to type 2 and 48% to type 3 following 2 doses 8 weeks apart. Among the 40 triple seronegative children, the seroconversion rates after 2 doses were 28% to type 1, 77% to type 2 and 40% to type 3. These rates are very poor, especially when compared to seroconversion rates in children in temperate climates. Loss of potency of the vaccine, interference of enteric viruses prevalent at the time with vaccine viruses and interference among the 3 vaccine virus types have been excluded as the cause of the poor serologic response observed in these children. Results of this and other similar studies should be taken into account when formulating dosage schedules of OPV in the tropics.