The presence of maternal antibodies is one of the main causes of measles vaccine failure. To evaluate the interference of passively acquired antibodies with vaccine efficacy, macaques (n = 16) were vaccinated with live attenuated measles vaccine in the presence or absence of passively acquired measles virus-specific monkey serum antibodies. As little as 0.1 IU of virus-neutralizing antibody/mL of serum abrogated the induction of specific serum IgM, IgG, and virus-neutralizing antibodies. This effect was also demonstrated in monkeys vaccinated with live recombinant vaccinia virus expressing the hemagglutinin and fusion proteins of measles virus but not in monkeys vaccinated with the same proteins incorporated into immune-stimulating complexes. All of the monkeys vaccinated in the presence of virus-neutralizing antibodies (n = 9) were still largely protected from intratracheal challenge with wild type virus. This protection is probably mediated by the observed specific T lymphocyte responses.