Changes in concentrations of IgM and IgG antibodies to Brucella were monitored for at least 13 mo by an enzyme-linked immunosorbent assay (ELISA) in 52 patients with culture-positive brucellosis. Two main patterns were observed. After an initial peak, 29 patients (56%) had a steady drop in their IgG levels, whereas 23 (44%) had more than one peak over time. All patients with a chronic form of brucellosis or a relapse were in the second group. In most cases, Brucella antibodies, although falling to low levels, remained measurable. Cutoff levels for IgM and IgG were calculated after considering serum antibody concentrations in people who had recovered from an infection. A separate normal range was established for occupationally exposed workers. On admission, sera from all patients contained Brucella antibody levels greater than established cutoff levels. Our results show that ELISA is an excellent method for diagnosis and follow-up of brucellosis.