Humoral immune responses were evaluated in a group of patients with acute brucellosis due to Brucella melitensis biotype I. Levels of immunoglobulins, C3 and C4, and the presence of antinuclear antibodies, rheumatoid factor and circulating immune complexes were sequentially studied in all brucellosis patients during the acute phase of the disease and following specific therapy. Elevated levels of all immunoglobulin classes, IgG, IgM and IgA and C3, C4, were found in the majority of patients. Antinuclear antibodies, rheumatoid factor and circulating immune complexes were found in circulation, particularly during the active stages. These findings provide support for the participation of the humoral immune system in the pathogenesis of human brucellosis.