We compared the immunological responses of leukocytes taken from healthy negative controls, laboratory workers immunized with the phenol-insoluble French vaccine against brucellosis, patients acutely ill with brucellosis, and patients chronically infected with Brucella melitensis. A salt-extractable antigen (protein-rich but with traces of lipopolysaccharide) and a sonicated suspension from B. melitensis 16M were used as antigens for in vitro lymphocyte proliferation test. Quantitation of T cells showed that the ratio of CD4+/CD8+ cells decreased as the condition of the patient deteriorated. An assay to quantitate the cell-mediated immunity showed that the activities of mononuclear cells stimulated with concanavalin A increased as the disease progressed as well.