The most important factor affecting the success of measles immunization is the disappearance of maternal anti-measles antibodies. In order to determine the optimum age for measles vaccination and to contribute towards the Expanded Programme on Immunization as currently applied in Turkey, we investigated the rate of disappearance of anti-measles antibodies. The study population consisted of 124 healthy infants aged 1-15 months from Erzurum, Erzincan, and Kars. The overall proportion of seropositivity, which is the result of the presence of maternal anti-measles antibodies, was 67/124 (54 per cent). The proportion of infants with detectable antibodies declined progressively with increasing age. The distribution of maternal antibody levels with respect to age showed a progressive reduction with increasing age from 7 months to 15 months. Thus the proportion of antibody-positive infants declined from 50 per cent at 7-9 months to 10 per cent at 13-15 months. While an evident decrease occurred during these months, no important decline was observed up to 9 months of age. The results of this study show that the minimum proportion of antibody-positive infants (10 per cent at 13-15 months of age) is still higher than the optimum proportion (5 per cent). The Schwarz vaccine, which is used mostly in measles immunization, seems not to be effective to obtain a high seroconversion rate in our infants. Edmonston-Zagreb vaccine strain should be given to children under the age of 15 months in eastern Turkey. In addition, serological studies should be performed periodically, and vaccination programmes appropriate for our country should be determined according to these data.