Mononuclear cells derived from bone marrow, synovium and peripheral blood of patients with rheumatoid arthritis (RA) were examined for their capacity to produce rheumatoid factor (RF) in order to investigate the origin of circulating RF. The results demonstrate that mononuclear cells derived from bone marrow are able to produce IgG-, IgA- and IgM-RF and that the amounts of RF produced by bone marrow cells are not significantly different from that by dissociated synovial cells. Since circulating immunoglobulins are mainly derived from the bone marrow this observation suggests that also RF circulating in RA patients mainly originates from the bone marrow.