The appearance of antibodies and development of immunity to measles was observed in 19 children in whom signs of measles had been prevented, during the course of epidemics, by the administration of gamma globulin. The epidemics occurred in two cottages of an institution for mentally retarded children. Both virus-neutralizing and complement-fixing techniques were used for the serologie tests, and human infant kidney tissue cultures were used for virus isolation. Of 38 susceptible children exposed to measles under these conditions and given gamma globulin, 19 developed measles antibodies without overt signs of the disease. Five of these 19 children failed to contract measles when exposed again seven months later.