ABSTRACT: The suppressive activity of human seminal plasma was confirmed on the luminol-amplified chemiluminescent reaction of human polymorphonuclears. This suppression was found to be dose-dependent and noncytotoxic. The responsible factor appeared to be a radical scavenger of low molecular weight, acting selectively on hydrogen peroxide. Correlation with glandular markers of semen indicated this molecule to have a prostatic origin. Neither crude seminal plasma nor this factor were found to suppress intracellular engulfment and killing of bacteria. This molecule seems to cooperate with the other semen antioxidants to protect sperm cells from extracellular oxygen radicals.