The introduction of live attenuated measles vaccine in Israel during 1967 dramatically decreased the incidence of measles. However, cases still occur in periodic outbreaks and epidemics, with an increasing proportion of infants and children younger than 2 years of age. We examined the decay of maternally derived measles antibody during the first year of life in the Jewish population of Israel which represents a highly vaccinated population with immunization rates exceeding 90%. We used sera of healthy full term infants born in 1988 and 1989. Fifty specimens for each of the following age groups were used: 0 (cord blood), 2, 4, 6, 7 and 12 months. Three assays for each specimen were used: enzyme-linked immunosorbent assay (ELISA); hemagglutination-inhibition test (HI); and neutralization test (NT). Good correlation among all 3 tests was found. All cord blood specimens were positive by at least 2 assays. Seropositivity rates declined rapidly with age. Fifty percent of all 4-month-old infants and < 30% of all 6-month-old infants were positive by 1 test or more; at 12 months of age none of the tested specimens was positive by HI or NT and only 1 of 50 infants was positive by ELISA. In infants younger than 6 months of age, 5 (22%) of 23 specimens negative both by ELISA and by HI were positive by NT, but in 6-month-olds, only 2 (7%) of 28 negative by ELISA and HI were positive by NT, and in 12-month-olds none was positive. The results from southern Israel are similar to those obtained in North America and provide evidence that infants older than 6 months of age in a well-immunized population may be poorly protected against measles. On the basis of this information and epidemiologic data, the Israel Ministry of Health has recommended lowering the immunization age for measles, mumps and rubella from 15 months to 12 months.