Live, attenuated mumps virus vaccine (Mumpsvax) was administered to 146 school children 6 to 9 years of age. One child developed clinical mumps nine days after vaccination; epidemiological and serological data strongly suggest that this child had become infected before vaccination. Apart from this single instance there were no apparent clinical reactions that could be ascribed to the administration of the vaccine. Sixty-three of the 146 children with no clinical history of mumps had an initial serum neutralizing antibody titre of less than 1:2. Specific antibodies to mumps virus were detected in 93.5% of the sera of the susceptible children 28 days after vaccination, and the geometric mean antibody titre of these sera was low (1:6). Of the 80 initially seropositive children 21 (26.2%) showed a significant antibody response to the vaccine and this was influenced by the pre-existing antibody level. These data have further demonstrated the safety and efficacy of the live mumps vaccine in children.