We report a secreted serine protease inhibitor Kazal-type-like (SPINKL) protein. The SPINKL protein was purified from mouse seminal vesicle secretions through a series of steps, including ion-exchange chromatography on a diethylaminoethyl-Sephacel column, gel filtration on a Sephadex G-75 column, and ion-exchange HPLC on a Q strong anion exchange column. Further analysis identified several SPINKL proteins with various N-linked carbohydrates. The SPINKL protein has six conserved cysteine residues that are nearly identical to those of members of the SPINK protein family. It was noted that the SPINKL protein showed no inhibitory activities against common serine proteases such as trypsin, chymotrypsin, subtilisin, or elastase. Spinkl mRNA and SPINKL proteins were found to be primarily expressed in seminal vesicles. Immunohistochemistry revealed that the SPINKL protein occurred in the luminal fluid and mucosal epithelium of the seminal vesicles and was regulated by testosterone. The SPINKL protein was able to bind onto sperm and enhance sperm motility. Also, it was able to suppress BSA-stimulated sperm capacitation and block sperm-oocyte interactions in vitro, suggesting that SPINKL may be a decapacitation factor.