To evaluate persistence of measles antibody after revaccination, antibody levels were measured 6 years after revaccination of 40 hemagglutination-inhibition (HAI) antibody-negative students who had participated in a serosurvey in Massachusetts. Twelve subjects who had been HAI antibody-positive and were not revaccinated were included as a comparison group. Before revaccination, 7 revaccinees had no detectable plaque reduction neutralization (PRN) antibody (group 1) and 33 had low levels of PRN antibody (group 2). Three weeks after revaccination, all in group 1 and 30 (90%) of 33 in group 2 had developed a fourfold or greater rise in PRN antibody. Six years after revaccination, all subjects had PRN-detectable antibody. However, 12 in group 2 (36%) had antibody titers less than or equal to 1:120 compared with none in group 1 (P less than .01). Persons without PRN antibody will respond to revaccination and maintain protective antibody titers. In contrast, persons with low levels of PRN antibody may respond initially to revaccination, but their antibody titers may fall again to low levels.