The healing of parietal and visceral peritoneum has been studied by the techniques of enzyme histochemistry in an attempt to define more precisely the type of cell responsibile for forming the new mesothelium. The changes occurring in the distribution of the following enzymes throughout the course of healing have been investigated: acid phosphatase, alkaline phosphatase, ATPase and non-specific esterase. Regenerating mesothelial cells have been found to have several enzyme histochemical properties in common with subperitoneal connective tissue cells. It has not been possible to distinguish between primitive mesenchymal cells and subperitoneal fibroblasts by the histochemical techniques used in this study and therefore the study has not been fruitful in determining whether the new mesothelium arises from primitive mesenchymal cells or subperitoneal fibroblasts. The present study does, however, lend weight to the view that the new mesothelium is not derived from peritoneal macrophages.