Ultrastructurally and enzyme histochemically two types of activities of Sertoli cells are to be proved. By means of closest membrane contact and intensive energetic changes of substances, the active cells participate largely in the ripening of spermatozoa. Steroid biosynthesis in Sertoli cells is possible according to their pattern of organelles and enzymes. The ultrastructure of Charcot-Böttcher’s crystalloids and the other characteristic inclusions in the cytoplasma of the Sertoli cells as well as implications to their function are describ