Human semen contains very potent blood-clotting activity; for example, seminal serum diluted up to 10 000-fold significantly decreased the recalcification clotting time of blood plasma. This seminal coagulant activity was dependent on factor X and calcium ions, suggesting the presence of a factor X activator. Immunoblotting analysis and immunoadsorption studies confirmed the presence of tissue factor antigen (45 kDa) in semen. Centrifugation studies suggested that tissue factor was membrane associated, and fractionation of seminal serum by gel filtration followed by immunoelectron microscopy revealed that tissue factor antigen was on the prostasome vesicle surface. Tissue factor originated from prostatic fluid and not from seminal vesicle secretions. Tissue factor antigen averaged 21 ng/ml in seminal serum. Hypothetical roles for very high levels of tissue factor in semen include several possibilities. In the event of abrasion and bleeding during intercourse, rapid blood clotting at lesion sites would prevent sperm and seminal components, including infectious agents such as human immunodeficiency virus, from entering the blood stream, generating antibodies, or promoting infectious disease. This could imply that development of infection from semen-borne agents or development of antisperm antibodies in some patients could result from impairment or absence of seminal tissue factor.