Humoral immunity to measles was studied in a group of children vaccinated before and a group vaccinated after their first birthday in the same office practice. All received live attenuated measles vaccine with measles immune globulin (MIG). The average postvaccination interval was five years. A significantly higher percentage of those vaccinated before 12 months of age were without detectable hemagglutination-inhibition antibody. Excessive dosage of MIG was not a significant factor in determining the lack of immune response. Revaccination of 12 seronegative children elicited an anamnestic antibody response in seven. The adjusted seroconversion rates, considering the anamnestic response to revaccination, were 79% and 100% for children vaccinated before and after their first birthday, respectively. These data support the recent recommendation that all children vaccinated before 9 to 10 months of age be routinely revaccinated.