The effectiveness of an oral polivirus vaccine using Sabin strains was studied among some 350 preschool children beginning in January, 1960. Although 79% of the children had previously received 3 or more doses of killed virus vaccine, a relatively high per cent lacked antibody to one or more types, and 22% of 348 children were triple negatives. The oral vaccine was given according to 3 different dosage schedules: each type singly; Type 1 followed by Types 2 and 3 together; and trivalent vaccine twice, 6 weeks apart. The infection and antibody conversion rates were similarly high for all 3 schedules, the overall conversion rates being 95% for Type 1, 98% for Type 2, and 86% for Type 3. The results emphasize the desirability of oral vaccine for young preschool children regardless of previous administration of multiple doses of killed Salk-type vaccine.