Abstract
Sections of rat organs were incubated upon gelatin films to which an azo dye had been attached. The films were dissolved at points where they were attacked by enzymes released by renal proximal convoluted tubules, thyroid follicular epithelium, serous tubules of the submandibular gland and macrophages. These structures could be identified by staining the sections as they lay upon the films. The resolution was sufficient to permit identification of individual active cells. The proximal tubule enzyme had a pH optimum of 4.4, had no essential sulfhydryl group and was active in the presence of a chelating agent.