This study evaluated the level of susceptibility of monocytes and lymphocytes to spontaneously induced and CH11-induced apoptosis in 16 patients with Brucella infection. The expression of some immunological and apoptotic markers was evaluated. Before therapy, monocytes showed a high level of resistance to spontaneously induced or CH11-induced apoptosis in all patients. In patients with acute infection, this resistance persisted for 10-20 days after treatment was initiated, then decreased; in chronically infected patients, it persisted after 45 days of treatment. Lymphocytes were also more resistant to CH11-induced apoptosis. The level of activated CD8(+) T lymphocytes was high in patients with acute infection. The data indicate that the CD95-mediated apoptotic pathway is not involved in CH11 resistance. Lymphocytes are not infected by Brucella, so their resistance to apoptosis may be due to a soluble factor released by infected monocytes. The evaluation of levels of susceptibility to CH11-induced apoptosis in monocytes may be used to test the effectiveness of the therapy.