ABSTRACT: Human seminal plasma has an anticomplement effect that can be measured by a standard immune hemolytic assay. We found that about 20% of samples lack complement-inhibiting activity. To determine the anatomical origin of a complement inhibitor, inhibition of hemolysis was measured in seminal plasma samples that were primarily of prostatic, vesicular, or epididymal/testicular origin, as well as in seminal plasma from vasectomized men. All samples contained complement-inhibiting activity, although epididymal fluid showed nearly twice the amount. Therefore, the factor (or factors) is ubiquitous in secretions of the male reproductive tract. Complement-inhibiting activity is eluted in two high-molecular-weight peaks upon gel filtration. We found evidence that the inhibitory factor is not lactoferrin, a proteinase, or one of the proteinase inhibitors known to be present in human seminal plasma. It seems likely that the complement inhibitor in seminal plasma protects gametes and reproductive tissues from complement-mediated damage.