By means of iron haematoxylin staining carefully differentiated by either picric acid or iron alum, Sertoli cells may be differentiated into two types, one (type A) containing unstained nuclei and the other (type B) intensely stained nuclei. No intermediate forms appear to be present. The two types are most easily found in the Sertoli-cell-only syndrome but they are present in the testes with spermatogenesis although type B is rare. The immature (pro-) Sertoli cells in infantile testes also display the two types. The two types of Sertoli cells differ in their degree of binding between nucleic acids and nucleic protein. The significance of the existence of these two types is discussed.