Innate immunity is important for the integrity of the host against potentially invasive pathogenic microorganisms in the environment. Antibiotic peptides with broad antimicrobial activity are part of the innate immune system. We investigated the presence of the cathelicidin, human cationic antimicrobial protein (hCAP-18), in the male reproductive system. We found strong expression of the hCAP-18 gene by in situ hybridization and hCAP-18 protein, as detected by immunohistochemistry, in the epithelium of the epididymis, but not in the testis. The highest expression in the epididymis was in the caudal part. Western blotting showed a doublet band, the upper part corresponding to the size of hCAP-18 in plasma and neutrophils. Using a specific enzyme-linked immunosorbent assay (ELISA), levels of 86.5 +/- 37.8 microg/ml (mean +/- standard deviation; range, 41.8 to 142.8 microg/ml; n = 10) were detected in seminal plasma from healthy donors, which is 70-fold higher than the level in blood plasma. Flow cytometry and immunocytochemistry revealed the presence of hCAP-18 on spermatozoa. ELISA measurement showed levels of 196 ng/10(6) spermatozoa, corresponding to 6.6 x 10(6) molecules of hCAP-18 per spermatozoon. Our results suggest a key role for hCAP-18 in the antibacterial integrity of the male reproductive system. The attachment of hCAP-18 to spermatozoa may implicate a role for hCAP-18 in conception.