A school blood drive before a measles outbreak permitted correlation of preexposure measles antibody titers with clinical protection using the plaque reduction neutralization (PRN) test and an EIA. Of 9 donors with detectable preexposure PRN titer ⩽120, 8 met the clinical criteria for measles (7 seroconfirmed) compared with none of 71 with preexposure PRN titers >120 (P < .0001). Seven of 11 donors with preexposure PRN titers of 216–874 had a ⩾4-fold rise in antibody titer (mean, 43-fold) compared with none of 7 with a preexposure PRN titer ⩾1052 (P < .02). Of 37 noncases with preexposure PRN titer P = .10). The study suggests that PRN titers ~120 were not protective against measles disease and illness without rash due to measles may occur in persons with PRN titers above this level.