Two hundred and twenty tree children were studied regarding their response to the measles vaccine. The antibody response was measured by hemaglutination inhibition technique. Among the variables studied only the children's age at the time the vaccine was done had influence on the seroconvertion. Sex and nutritional status had no influence on the seroconvertion. A theoric model was designed to find at what age the vaccine had the best effect in preventing mortality. The age predicte was seven months.