1. Approximately 1000 Nigerian children between the age of 6 months and 2 years were given Enders ‘B’ meásles vaccine, combined with gamma globulin in all except 120 children. A control group of similar numbers received gamma globulin alone plus inert material.2. In a preliminary village study the 26 vaccinated children remained free of measles, 19 of the 27 control children developed measles and 2 of these children died.3. In a blind study among children attending the large child out-patient clinic at the Ilesha hospital, a follow up was also attempted but was more difficult. In all, 272 cases of ‘measles-like’ illness were seen, 192 in control children, and 80 in the vaccinated group. The number in the vaccinated group is believed to be a reflexion of the difficulties under which this diagnosis was made, rather than a failure of the vaccine to immunize. Eleven children are known to have died from measles. All these 11 were in the control group who had not had vaccine.4. The control children showed a smaller mean gain in weight than the vaccinated in the months following inoculation. The difference may have been due to poor weight-gain among the control children who developed measles.This work was supported in part by the Health Research Council of the City of New York under contract U 1056 and by the office of the Surgeon General, Department of the U.S. Army, sponsored by the Commission on Viral Infection Armed Forces Epidemiological Board. We wish to thank Dr W. J. Martin, Ph.D., D.Sc. and Miss I. Allen of the Medical Research Council's Statistical Unit for Statistical help.The vaccines and gamma globulin were supplied by Dr Maurice Hilleman, Director, Merck Institute for Therapeutic Research.