['To examine the effects of a fibronectin (FN) fragment containing the COOH-terminal heparin-binding domain (HBFN-f) on chondrocyte-mediated type II collagen (CII) cleavage by collagenase and proteoglycan (PG) degradation in articular cartilage in explant culture.', 'Intact FN or HBFN-f was added to explant cultures of mature bovine articular cartilage. We investigated collagenase-mediated cleavage of CII caused by HBFN-f in explant cultures using a new immunoassay for detection and measurement of the primary collagenase cleavage site of CII. CII denaturation in cartilage was also measured using a specific enzyme-linked immunosorbent assay. Degradation of PG (principally aggrecan) was analyzed by a dye-binding assay. APMA and/or a matrix metalloproteinase 13 (MMP-13) preferential inhibitor or interleukin-1 receptor antagonist (IL-1Ra) were added to some cultures to examine the presence of latent procollagenase or the involvement of MMP-13 or IL-1, respectively, in cartilage breakdown induced by HBFN-f. Secretion of MMP-3 and MMP-13 into media was detected by immunoblotting.', 'In contrast to intact FN, HBFN-f was shown to stimulate CII cleavage by collagenase in a dose-dependent manner following PG degradation, similar to cartilage breakdown induced by IL-1. Treatment with HBFN-f also resulted in elevated denaturation of CII. Immunoblotting demonstrated that HBFN-f enhanced pro-matrix metalloproteinase 13 (proMMP-13) production as well as that of proMMP-3. APMA, which activates latent proMMPs, enhanced the HBFN-f-mediated cleavage of CII by collagenase. An MMP-13 preferential inhibitor or IL-1Ra suppressed HBFN-f-induced collagen cleavage to control levels.'] Does a fibronectin fragment induce type II collagen degradation by collagenase through an interleukin-1-mediated pathway?