A concentrated, alum-containing, formalin-inactivated measles virus vaccine was used in a double-blind field trial conducted among school children. The dosage schedule consisted of 0.5 ml. of either vaccine (330 children) or placebo (319 children), intramuscularly, followed by a second such dose a week later, and a final injection 21 days after that. Most children completed their series on schedule. Subsequent observation disclosed a decrease in measles of 96% if only cases noted 14 or more days after the third dose were considered. If all cases that occurred after the first dose were counted, then the protection amounted to about 65%. The vaccinated group had a marked decrease in school absenteeism.