Quantitative serologic responses following the inoculation of infants less than one year of age with live, further-attenuated measles virus vaccine were compared to those of infants and children inoculated after one year of age. Active/passive immunization resulted in reduced antibody formation in some infants especially those less than 9 months of age. Thirty-seven infants identified as "vaccine failures" following their initial inoculation at less than one year of age were revaccinated after one year of age. Fifty-one percent had no detectable HI antibody by eight months postrevaccination, contrasted to 6.8% of the vaccinees with no detectable HI antibody following one inoculation after one year of age; 49% responded optimally to revaccination. In face of an observed altered response in many infants less than one year of age, it would appear prudent to withhold vaccine in this age group until the consequences of such an approach are better defined.