Because the Wistar RA27/3 strain rubella virus vaccine has potentially important advantages over rubella vaccines currently licensed in the United States, a field trial was conducted in Minnesota and Wisconsin in 1974 to evaluate RA27/3 in this country. Two hundred eighty-five (99.7%) of 286 susceptible children given RA27/3 subcutaneously seroconverted, with a geometric mean hemagglutination inhibition (HI) titer of 81.2. Twenty-eight (23%) of 122 children with rubella antibodies before immunization had fourfold or greater rises in rubella HI titers. The highest percentage of booster responses occurred in children with low preimmunization titers. Side effects were reported in 34% of subjects, but only one reaction associated with RA27/3 was serious: in a 5-year-old boy, arthritis of the left hip developed 31 days after immunization. This study indicates that RA27/3 vaccine produced a very high rate of seroconversion with high postimmunization HI titers. The ability to elicit significant booster responses in children with low levels of HI antibodies suggests that RA27/3 could be used to boost immunity in women of childbearing age whose rubella titers have declined to undetectable levels.