Abstract A function and identity of a 24,500 Da protein derived from round spermatids of the rat testis was investigated with a specific polyclonal antiserum raised against RSP-24.5. The proteins released from cultured round spermatids significantly stimulated the secretion of de novo synthesized protein from cultured immature rat Sertoli cells 2.4-fold above control levels. Immunoprecipitation of RSP-24.5 from round spermatid protein further enhanced the stimulation of Sertoli cell protein secretion up to 3.1-fold above control levels, indicating that RSP-24.5 plays a role in the down regulation of Sertoli cell secretion. The antiserum recognized the 24,500 Da protein in Western blots of round spermatid protein, pachytene spermatocyte protein, Sertoli cell lysate and peritubular myoid cell lysate. A 40 amino acid sequence of a cyanogen bromide cleaved internal fragment of RSP-24.5 showed 80.5% homology to a phosphatidylethanolamine binding protein. These results suggest that phosphatidylethanolamine binding protein participates in the negative regulation of Sertoli cell secretory function during spermatogenesis.