Although measles vaccine has been licensed since 1963 the disease has not yet been eradicated. One of the obstacles to successful immunization is th presence of maternal antibody; previous studies have shown that low levels of passively acquired maternal antibody may actually inhibit the developmen of a protective immune response following immunization with live attenuated strains of measles vaccine virus. These studies were done at a time when infants were born to mothers who had undergone natural infection with wild type measles virus. Currently, an increasing number of infants are born to mothers who have received inactivated or live attenuated measles virus vaccine. In order to determine the effects of maternal antibody for infant born to immunized mothers a study was done to determine the level of maternal antibody for three groups of infants: (1) infants born to older mothers who had been infected with wild type measle virus; (2) infants born to mothers who had received inactivated and live attenuated measles vaccine and (3) infants born to mother who had received only live attenuated measle virus. In addition the immune response to two doses of measles vaccine (monovalent Schwarz vaccine given at 6 months and MMR containing the Morate strain of measles vaccine given at 15 months) was determined for infants in the three groups. As expected, infants in Group 1 had the highest levels of maternal antibody and low seroresponse rates to measles immunization at 6 months (37% seroconverted). In contrast, infants born to Group 2 and 3 mothers had low levels of maternal antibody and 81 and 85% seroconverted after monovalent measles vaccine at 6 months. After a second immunization at 15 months 97%, 100% and 97% of the children in groups 1,2, and 3 respectively had protective levels of measles virus neutralizing antibody. The data indicate that infants born to immunized mothers may be effectively immunized at a younger age than infants born to mothers who have undergone infection with wild type measles virus.