ESc-615 is a gene which was specifically expressed in epididymis, newly discovered in our laboratory. To elucidate the function of ESc-6 15 in sperm maturation and fertilization at the protein level, its C-terminal 310 amino ac id residues fragment was expressed in E.coli and used to immunize rabbits to obtain polyclonal antiserum with titer of 100 000. The antisera could detect th e antigen as low as 3 ng. A 63 kD protein in rat epididymis protein extracts could be detected in Western blot by using this particular antiserum, in accordance with the size of Esc-615 homologue polypeptide deduced from its cDNA. Immunostaining results showed that Esc-615 was a secreted protein and could bind sperm s, which was similar with other carboxylesterases in male reproductive system; t he positive staining was eliminated after antigen absorption. All these results showed that this antiserum was not only effective but also highly specific, providing a useful tool in further functional studies of Esc-615.