Bone marrow cells from patients with multiple sclerosis (MS) were studied regarding proliferative capacity with and without mitogenic stimulation, immunohistochemical characterization of cellular phenotypes with monoclonal antibodies and morphology and compared to bone marrow cells from healthy individuals undergoing elective orthopaedic surgery. MS patients' bone marrow mononuclear cells (BM-MNC) showed higher spontaneous proliferation both in comparison with BM-MNC from controls and peripheral blood mononuclear cells (PBL) from MS patients. Phytohaemagglutinin (PHA) response was higher in MS BM-MNC than BM-MNC from controls. MS patients' BM-MNC proliferated more on interleukin-2 (IL-2) stimulation than their corresponding PBL. There was no significant difference in proliferative response of PBL between MS patients and controls. Higher levels of undifferentiated or activated cells, as measured by OKT10, were found in peripheral blood of patients with MS. Seven of 11 MS patients showed morphological signs of activation in their bone marrow. The results indicate a role for immune reactions in bone marrow in the pathogenesis of multiple sclerosis, a disease with symptoms and signs strictly confined to the central nervous system.