Osteoarthritis (OA) is characterized by a progressive erosion of cartilage, which is mediated by the protease degradation of the extracellular matrix components. Plasmin, plasminogen activators (PA) and the inhibitor of plasminogen activator (PAI) are thought to play an important role in the regulation of the OA pathophysiology process. Our study determined the activity of plasmin and PA in OA and normal cartilage. Moreover, the presence and the content of each form of PA, uPA and tPA, as well as the inhibitor PAI-1, were determined using immunohistological techniques and ELISA. Our studies were carried out on fresh cartilage, cultured tissue explants and chondrocytes. OA cartilage demonstrates about 5 times more plasmin activity than the controls (p less than 0.001). Moreover, a statistically significant correlation was found between the plasmin activity and the free collagenolytic form in OA specimens showing severe lesions (r = 0.50; p less than 0.05). Our study revealed that PA content and activity increase in OA cartilage following culture explant experiments. Immunohistochemical studies showed the presence of both uPA and tPA forms in OA cartilage lesions only. Protein determinations revealed uPA as the predominant form. PAI-1 was significantly decreased (p less than 0.04) in OA, and was located mainly extracellularly. Chondrocyte cultures showed the ability to synthesize both forms of PA and PAI-1. Our study demonstrated an increased level of plasmin activity in OA cartilage. This is likely related to increased PA activity, in which the urokinase type appeared to be predominant in OA.(ABSTRACT TRUNCATED AT 250 WORDS)