A METHOD for the histochemical demonstration of active carbonyl groups of lipoid and non-lipoid material has been described by us (Ashbel and Seligman, 1949; Seligman and Ashbel, 1949; Seligman, Friedman and Herz, 1949). After formalin fixation of the tissue, it was possible to demonstrate active carbonyl groups in some of the lipoid of various endocrine glands such as adrenal cortex, corpus luteum, testicular interstitial cells, and syncytial trophoblasts of placenta (Ashbel and Seligman, 1949). Reasons for considering the material thus demonstrated to be ketosteroid have been discussed (Ashbel and Seligman, 1949) and more evidence for this claim appears below. Material, which is found in the white matter of the central nervous system, in the myelin sheath of peripheral nerve, and in other tissues, is poorly soluble in lipoid solvents, contains carbonyl groups after fixation in formalin (Seligman and Ashbel, 1951),