A cDNA clone encoding androgen-dependent proteins secreted by the mouse epididymis was isolated by screening a cDNA library using differential hybridization, according to the selective expression of the mRNA in the normal but not in the castrated mouse. Translation of mRNA hybrid-selected by this 1·4 kb clone (M53) yielded proteins of Mr 26 000 which were processed in vitro in the presence of microsomal membranes into proteins of Mr 24 000. Northern blot analysis of epididymal total RNA revealed at least two populations of mRNA (1·4 and 1·8 kb) homologous to the M53 clone, which were restricted to the caput epididymidis. Studies in vivo demonstrated that testosterone regulates the concentration of these mRNA populations. Analysis of epididymal total RNA from ten individual animals provided no evidence that the M53 mRNA populations are the products of allelic variants of a gene. Southern analysis of mouse genomic DNA revealed single bands with most of the tested restriction enzymes. Furthermore, cross-hybridization to the M53 cDNA revealed homologous mRNA species in rat, human, rabbit, ram and boar epididymal RNA.