OBJECTIVE:To search for the presence of platelet-activating factor (PAF) acetylhydrolase activity in human seminal plasma.DESIGN:Experimental.SETTING:Reproductive laboratory in a university-affiliated hospital research center.PARTICIPANTS:Human male volunteers were selected on the basis of apparent normal health.RESULTS:Human seminal plasma contained significant levels of PAF-acetylhydrolase activity. Enzymatic hydrolysis of PAF displayed typical kinetics and was a calcium-independent process. Platelet-activating factor acetylhydrolase in seminal plasma was associated with a very high-density lipoprotein fraction. Enzymatic activity was independent of sperm count and motility.CONCLUSION:Human seminal plasma contains significant levels of PAF acetylhydrolase. This enzyme may be an important factor in the regulation of PAF concentration and activity.