A concentrated, alum-containing, formalin-inactivated measles virus vaccine was evaluated in a double-blind field trial conducted in young school children. Vaccine was administered at random to 296 five- and six-year olds and placebo to 286. The dosage schedule consisted of three intramuscular injections of 0.5 ml of vaccine or placebo at intervals of one and three weeks. Measles was prevalent in the study population during the immunization period and for three months thereafter as well as from the 18th through the 27th months of follow-up. During the immediate postvaccir ation challenge vaccine effectiveness was estimated at 94% while during the late postimmunization challenge, effectiveness had declined to 57%. Nevertheless, during the latter period, the proportion of mild cases was considerably higher among vaccinees than controls.