This study on the regulation of interleukin (IL)-11 expression in human connective tissue cells shows that IL-11 expression is not restricted to cells of hematopoietic origin but can also be induced in articular chondrocytes and synoviocytes. IL-11 mRNA was induced in chondrocytes in response to transforming growth factor (TGF)-beta 1 and IL-1 beta. Stimulation with IL-6 or growth factors, such as basic fibroblast growth factor, leukemia inhibitory factor, and platelet-derived growth factor-AA, had only weak or no detectable effects. Activation of protein kinase C by phorbol esters and inhibition of protein synthesis by cyclohexamide increased IL-11 transcripts, whereas calcium ionophore A23817 or dibutyryl cyclic AMP had no effect. Immunoprecipitations revealed the synthesis of IL-11 protein in response to TGF-beta 1, IL-1 beta, as well as phorbol 12-myristate 13-acetate, and a synergistic action of TGF-beta 1 and IL-1 beta was observed. Similar findings on IL-11 expression were made in synoviocytes. Analysis of effects on cell function showed that IL-11 stimulated the production of the tissue inhibitor of metalloproteinases in chondrocytes and synoviocytes but did not affect chondrocyte proliferation or increase stromelysin activity. These results suggest that IL-11 does not contribute to connective tissue degradation but conversely induces protective effects in joint tissue.