OBJECTIVES To estimate the proportion of individuals seronegative for measles antibody among blood donors from a blood bank in the city of Rio de Janeiro, Brazil, and to describe their social and demographic characteristics, with the goal of exploring the potential use of plasma banks to supplement serological surveillance with relevant data in order to support the program of measles elimination in the city of Rio de Janeiro and elsewhere. METHODS Plasma samples from 1 101 consecutive blood donations made in November 2000 at HEMORIO, the largest blood bank in the state of Rio de Janeiro, were tested for measles immunoglobulin G, using a commercial enzyme immunoassay and a plaque reduction neutralization test. We calculated the proportion (and 95% confidence interval (CI)) of samples seronegative for measles antibody for the total sample of blood donors and also for subgroups categorized by age, sex, neighborhood of residence, education, and occupation. We used the chi-square test to assess the statistical significance of differences in proportions and linear trends in proportions. RESULTS Of the total group of blood donors, 6.9% of them (95% CI: 5.4%-8.4%) were seronegative for measles. Women had a higher proportion (10.1%; 95% CI: 6.8%-13.4%) of seronegative results than did men (5.6%; 95% CI: 4.0%-7.2%). In terms of age, 86.8% of seronegative individuals were born between 1971 and 1982. Seronegativity was inversely proportional to age (chi-square = 58.0; P < 0.0001). With regard to occupation, students had the highest proportion of seronegative individuals (17.8%). In terms of education, most of the susceptible persons were in the categories of "incomplete university degree" or "incomplete high school." For the various areas of residence the proportions ranged from 2.1% to 11.4%. CONCLUSIONS Blood bank plasma may constitute a useful and convenient source of complementary data for serological surveillance in adults of measles and other infections for which immunization and surveillance activities are implemented. This approach could be beneficial to other areas in Brazil and other countries where plasma from blood donors is available for surveillance. The use of residual sera from patients and plasma from blood donors represents a tradeoff between representativeness and timeliness as well as economy of resources.