This study demonstrates that interleukin-6 (IL-6) increases the autoantibody production by B cells from NZB/W F1 mice. Splenic B cells were cultured for 5 days in the presence or absence of human IL-6, and then the anti-DNA antibody and immunoglobulin contents in the culture supernatants were measured by ELISA. Adding IL-6 increased IgG anti-DNA antibody production by B cells from old mice (30 weeks), but not from young ones (17 weeks). B cells obtained from both young and old mice produced IgM anti-DNA antibody, which increased when IL-6 was added. The increased anti-DNA antibody production was suppressed by anti-recombinant human IL-6 antibody to the background level, i.e. antibody contents in the absence of IL-6. In contrast, murine IL-5 did not increase IgG anti-DNA antibody production, although it promoted the production of IgM anti-DNA antibody. Furthermore, when IL-5 was added in combination with IL-6, there was an additive increase in IgM, but not in IgG anti-DNA antibody production. Similar results were obtained in the measurement of the immunoglobulin contents. These results suggest the possible role of IL-6 in the pathogenesis of autoimmune disease in NZB/W F1 mice.