In a coculture with porcine articular cartilage explants unstimulated blood mononuclear cells (BMC) from patients with rheumatoid arthritis (RA), but not from healthy controls, induced proteoglycan depletion of dead cartilage. Specific stimulation of the RA BMC with Mycobacterium tuberculosis (MT), in comparison with concanavalin A (Con-A), strongly enhanced the proteoglycan depletion of living cartilage; this was not found with the BMC of healthy controls. However, the MT induced proliferative responses of the same BMC were similar in healthy controls and patients with RA. Neither the proliferative response nor the proteoglycan depletion was influenced by the presence of HLA-DR4 in the donor, whether patients with RA or healthy control. The proliferative responses of the RA BMC seemed to correlate inversely with the proteoglycan depletion. We conclude that stimulation of RA BMC with mycobacterial antigens may elicit effector pathways that induce proteoglycan depletion, independent of T cell proliferation.