['Testin I and Testin II were originally identified as Sertoli cell products with similar NH2-terminal amino acid sequences. Secretion of testins is stimulated by testosterone in Sertoli cell-enriched cultures. By contrast the secretion of testins from intact seminiferous tubules appears to be inversely related to germ cell number. In the present study testin antiserum that recognized both Testin I and Testin II ("testin") was used to localize these proteins in tissue secretions by immunofluorescence. Testin was localized at the base of the seminiferous epithelium at Sertoli-Sertoli junctions. Fluorescence also appeared to be located at the sites of interaction between spermatoids and Sertoli cells. A punctate pattern of fluorescence was also present in the cytoplasm of Leydig cells; without electron microscopic studies it was not possible to determine which structures the antibodies bound to in these cells. In the epididymis the reaction product was localized at the apices of the epithelial cells adjacent to the lumen at the sites of known junctional complexes. A variety of positive and negative controls indicated that staining was specific for testins.'] Are testins localized to the junctional complexes of rat Sertoli and epididymal cells?