This paper reports experiences with a concentrated, alum-containing, formalininactivated measles virus vaccine. The Edmonston (Enders) strain, cultivated in monkey kidney tissue culture cells, was used. Three doses of 0.5 ml. of vaccine were administered intramuscularly to alternate children in 22 families. The other children received placebo injections. Reactions were minimal in both groups. Neutralizing antibodies appeared in 72% of the 18 children who received 3 doses. Those who acquired antibodies and were exposed to measles remained quite well while their placebo-treated siblings became quite ill. On subsequent challenge with live, attenuated measles virus vaccine, the immunized children remained well, even if their antibodies were no longer detectable, while the placebo group had some symptoms; both had excellent serological responses.