Interleukin 1, a product predominantly of monocytes, increases the synthesis and release of procollagenase and prostaglandin E2 by mesenchymal target cells such as synovial fibroblasts and articular chondrocytes, an effect mimicked by some phorbol esters. In order to determine the mechanisms underlying these responses primary cultures of human articular chondrocytes were preincubated with recombinant human interleukin 1 beta or the phorbol ester, phorbol 12-myristate 13-acetate, in the presence or absence of the cyclooxygenase inhibitor, indomethacin. Interleukin 1 beta or phorbol ester increased the levels of procollagenase (assayed after trypsin activation) and the labeling of several medium proteins by cells incubated with [35S]methionine, independent of prostaglandin synthesis. The labeling of a 55 kD protein immunocomplexed with antibodies to procollagenase was also increased. The increased synthesis of procollagenase was paralleled by increased cellular levels of procollagenase mRNA, determined with a cDNA probe coding for human procollagenase. Thus the increased synthesis of procollagenase in response to the inflammatory mediator, interleukin 1, is controlled at a pretranslational level, possibly at the level of transcription.