Measles antibody (HAI) titres were measured in healthy Nigerian infants at birth and at 3, 6 and 9 months of age. At birth only one baby out of thirty-five had HAI titre of less than 1:4, but at 3 months of age titres had fallen to this level in 33% of children. The percentage increased further to 78.6% at 6 months of age. At 9 months, nine out of twenty-seven (33%) infants had HAI titre of 1:16 and above, suggesting recent measles infection or immunization. The rapid fall in HAI titres up to 6 months could explain the occurrence of measles in early age in tropical countries. We recommend that children in Nigeria be immunized at 7-8 months of age.