Numerous studies both in the United States and abroad1-10have fully confirmed the original demonstration11,12of the efficacy of attenuated live measles virus vaccine derived from Enders' Edmonston strain.13Of the several approaches to the problem of reducing the severity of symptomatic reactions to the vaccine without impairing effectiveness, that of concurrent administration of human γ-globulin has received the most attention and support.1,4-10 This report summarizes the results of a controlled trial of live measles vaccine, with and without human γ-globulin, in 134 children six months of age and older. The results permit definition and quantitative comparison of the relative effects of γ-globulin and of various host factors—age, history of measles in siblings, and prevaccine antibody—on the symptomatic and immune responses to the vaccine. Although the results substantially confirm the efficacy of live measles vaccine, the data re-emphasize its unreliability in children under one year of