AbstractTwenty‐five leg ulcer exudate samples from 17 patients with chronic non‐healing venous leg ulcer were analyzed for proteolytic activity using radial caseinolysis procedures and zymographic analysis, and for fibronectin fragmentation using immunoblotting technology. Caseinolytic activity was detected in 21 of the 25 samples. A minority of them were inhibited (3 were totally, 6 partially inhibited) by aprotinin, a serine proteinase inhibitor, suggesting that proteinase(s) other than plasmin were also responsible for the caseinolysis. In zymographic analysis, 23 of the 25 samples showed positive reactions for enzyme activities comigrating with plasmin and urokinase‐type plasminogen activator. Fibronectin fragmentation, another sign of proteolytic activity, was seen in all but 2 ulcers. No correlation was seen between bacterial infection or inflammatory cells and the above parameters in the wound fluid. Acute wound fluid collected from the donor sites of patients undergoing split skin grafting was used as a control. In the control specimens no proteolytic activity was found during the days following operation. These results show that there is proteolytic activity in the chronic ulcer exudate and support the possibility that the proteolytic activity and consequent fibronectin fragmentation may be related to the retarded epithelization and ulcer he