There is still controversy about the optimum age for measles vaccination in developing countries, where the incidence of measles infection is higher in the first few months of life than it is in developed countries. This study was undertaken to collect reliable data in order to determine the optimum age for mass vaccination programmes. Haemagglutination inhibition (HI) antibodies were titrated periodically from birth to one year of age in children who were given the vaccine at different ages, between 5 and 9 months. It was found that 90% of children no longer have their maternal antibodies at 7-8 months of age, precisely at the period that the incidence of measles begins to rise sharply. Almost all children showed HI seroconversion when vaccinated at 71/2 months (or later, but not before), even if a low level of maternal antibody still persisted when the vaccine was given. These data show that there is an advantage in carrying out measles vaccination at 71/2 months of age in countries with conditions similar to that of Kenya. The duration of post-vaccinal immunity beyond one year of age has not been studied, but it can reasonably be expected that immunity after one vaccination can last for at least 3-5 years, thus exceeding the period when African children are most exposed to malnutrition.