It is well known that rat Sertoli cells in culture secrete both testis-specific proteins, such as inhibin and androgen binding protein (ABP), and proteins which are very similar, if not identical, to serum proteins, such as transferrin (TF), ceruloplasmin, and IGF-I. It is also well known that very few data have been reported about the secretory activity and the hormonal regulation of the Sertoli cell in man, mainly because of the difficulties associated with the isolation of pure cell populations from human tissue. Using histoimmunochemical techniques we tried to localize, with specific antisera, Sertoli cell proteins and, when possible, their receptors in the human testis. The results obtained with our Light Microscopy studies suggest that: (1) human Sertoli cells produce and/or store transferrin (TF), IGF-I, an albumin-like protein and ABP; (2) TF receptors are localized in spermatocytes and early spermatids and are absent in spermatogonia, in the cytoplasm of Sertoli cells and in differentiated spermatids; (3) IGF-I type I receptors are localized in the same germ cells and in the cytoplasm of Sertoli cells. The results obtained with our Electron Microscopy studies suggest that TF and IGF-I are internalized through a receptor mediated endocytosis mechanism both in Sertoli cells (basal compartment) and in germ cells (spermatocytes and early spermatids).