The natural decline of passively acquired maternal antibody titers and the influence of these antibodies on the efficacy of live, further attenuated measles vaccine were studied in 7 to 12 month old infants. Hemagglutination-inhibition (HI) antibody titers were determined in the pre- and postvaccination sera. HI antibodies at low titers were detected in 20 (46%) of the 43 infants investigated, in five of six infants at seven months and two of nine infants at 12 months of age. The serological conversion rate following vaccination of the infants exhibiting transplacental maternal antibody was as low as 38%, in contrast with 100% sero-conversion of the infants without measurable antibodies of maternal origin. HI antibody titers were low in the infants seven to nine months of age, reflecting poor antibody responses to the administered vaccine. For the 12 month old infants the geometrical mean antibody titer was 1 : 52. It is concluded that transplacental maternal antibody, even at low concentration, inhibits induction of HI antibody in the vaccinees and therefore measles vaccination must be initiated after 12 months of age in order to achieve successful immunization of the children. Infants who receive the vaccine before their first birthday have to be revaccinated at or after 15 months of age.