Beginning in January, 1962, two Arizona counties, Maricopa and Pima, conducted separate, but similar, voluntary mass immunization programs using Sabin oral polio vaccine. All clinics were scheduled on Sundays, and school buildings were used as clinic sites. The vaccine was dispensed on sugar cubes. During these programs, 754,000 persons were immunized against all three types of polio. This represented approximately 75% of the total population of both counties.