The results of classic serological tests were compared with those of enzyme-linked immunosorbent assay in studies of immunoglobulins to Brucella in 761 serum samples from 75 patients with brucellosis. Except for five instances involving the IgM ELISA, all serological tests gave positive results at admission. Among the 63 patients without relapse, rates of persistent ELISA positivity (determined by the Kaplan-Meier method) 12 months after therapy were 25% for IgM, 69% for IgA, and 89% for IgG. Among the 12 patients with relapse, a second peak of ELISA IgG and IgA was often detected. The persistence of high serum antibody titers in patients without relapse was due mainly to IgG and was often associated with high titers at admission or with the presence of focal disease. Overall, serological changes were better detected by ELISA than by classic serological tests. While a second peak of ELISA IgG and IgA is a good marker of relapse, the persistence of high titers of IgG by itself is not a good predictor of chronic infection.