The cytochemical profile of activated mesothelial cells differs from resting cells. The reaction products of enzymes associated with oxidative mechanisms of cell respiration were easily displayed in activated mesothelial cells; in resting mesothelial cells, only enzymes of the pentose pathway were readily demonstrable. Acid hydrolases were detected in greater quantity than in resting cells, possibly reflecting an increased potential for endocytosis. In addition, the cytochemical assays indicated increased Golgi activity was reflected by the demonstration of thiamine pyrophosphatase, while the content of ribonucleic acid was also increased. These cytochemical features compare well with the ultrastructure of active mesothelial cells which possess abundant ergastoplasm and a well-developed Golgi apparatus. In comparison with peritoneal macrophages, activated mesothelial cells differ mainly in the quantity of reaction product, there being more in macrophages. The results were significantly different only in the demonstration of lipids which were never found in mesothelial cells, but were invariably present in macrophages. It is still unclear whether the acid mucopolysaccharide hyaluronic acid is produced by or merely transported through the mesothelium from the subserosal site.