The expression and localization of the human sperm protein hCAP-18/SOB3 were evaluated in human testis and epididymis through in situ hybridization and immunohistochemistry with the use of an anti-recombinant hCAP-18/SOB3 polyclonal antibody. Both hCAP-18/SOB3 messenger RNA and hCAP-18/SOB3 protein were detected in testis germinal cells (from late spermatogonia to spermatozoa) and in the epididymal epithelium. This localization is in agreement with the antimicrobial properties previously described and supports its involvement in zona pellucida binding, as we had previously suggested.