Millions of Americans suffer from degenerative joint diseases that affect the temporomandibular joint (TMJ). In these affected individuals, the collagen degradation is mediated in part by TMJ synovial fibroblasts. These fibroblasts produce matrix metalloproteinases (MMPs) that are able to cleave multiple components of the extracellular matrix. Recently, a novel non-MMP collagen degrading pathway has been identified in a TMJ synovial fibroblast cell line after 8-13 passages in culture and is referred to as the aggressive phenotype because of their increased collagen degrading ability. In earlier passages, these fibroblasts displayed a less aggressive phenotype and are referred to as the non-aggressive phenotype. Objective: The specific aim of this project was to determine the rate of collagen cleavage mediated by the conditioned media from the aggressive and non-aggressive fibroblasts. Methods: Rat-tail-tendon Type 1 collagen was labeled with fluorescein isothiocyanate and mixed with unlabelled collagen to form collagen gels. Aggressive and non-aggressive fibroblast media was added to the collagen gels, incubated overnight, and then the fluorescence released from the collagen was determined and the rate of collagen cleavage calculated. Results: The amount of collagen cleavage mediated by the aggressive conditioned media was 5 fold more than that mediated by the non-aggressive conditional media. The collagen cleaving ability of the non-aggressive phenotype was effectively inhibited with 1,10-phenanthroline, an MMP inhibitor. The collagen cleavage mediated by the aggressive phenotype was more effectively inhibited by soybean trypsin inhibitor (STI), a serine proteinase inhibitor, than with 1,10-phenanthroline. Conclusion: The amount of collagen cleavage mediated by the aggressive cells was substantially more than that mediated by the non-aggressive cells. The ability of STI to more effectively inhibit the collagen cleavage mediated by the aggressive cells than that of an MMP inhibitor further demonstrated that the collagen cleavage mediated by the aggressive cells is serine proteinase(s) dependent.