We previously showed in a primary culture of rat testicular cells that spermatogenic cells specifically bind to somatic Sertoli cells and that this interaction is needed for spermatogenic cells to differentiate in vitro. Adopting an expression cloning procedure, we here isolated a cDNA coding for a spermatogenic cell protein whose expression gave a cultured cell line the ability to bind to Sertoli cells. The protein, 243 amino acids with a putative N-terminal signal peptide and a C-terminal Cys-rich region, turned out to be the rat homologue of a testicular protein called Tpx-1 whose function had yet to be determined. A polyclonal antibody raised against bacterially expressed Tpx-1 significantly inhibited the binding of spermatogenic cells to Sertoli cells. The above results indicated that Tpx-1 is a testicular cell adhesion molecule responsible for the specific interaction between spermatogenic and Sertoli cells.