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The measles-specific antibody responses of seronegative adolescents and young adults were evaluated after revaccination. Of 1650 previously vaccinated healthy volunteers between the ages of 10 and 30 years, 4.4% were found to be seronegative for measles antibodies and 9.9% had equivocal titers. Seronegative volunteers were revaccinated to measles and followed serially for development of measles-specific IgG. Of 43 subjects followed for at least 1 year, only 58% developed and maintained positive antibody titers; 12% never developed positive titers and 30% initially developed titers that fell below positive levels within 1 year. The peak titers achieved by those subjects who responded transiently were lower than those achieved by subjects who developed sustained responses. Thus even after the recommended two dose schedule of the current measles vaccine, some adolescents and young adults lack protective titers of measles-specific antibody.