Measles antibody titres were determined by haemagglutination inhibition and by neutralization in 221 sets of serum collected from delivering mothers, umbilical cords, and infants when about six months of age. Radio-immunoassay was also used to measure antibody in 120 sera. Total IgG concentration was determined in the infant sera. All mothers had measles antibody and the mean titre was high. At the time of birth, measles antibody had been further concentrated in the infant. Nevertheless, many children lost protective titres before six months of age. The rate of loss was correlated with the infant's total serum IgG so that high IgG levels at six months correlated with rapid loss of measles-specific antibody. It is suggested that in homes where sanitation is poor, antibody is made to many agents at an early age. To maintain physiological balance, homeostatic mechanisms then increase the rate of catabolism of all IgG, including that passively acquired. In keeping with its stage of sanitary development, vaccination in Jamaica can profitably be given earlier than in the United States, but it must be later than in many African countries.