Various coagulation abnormalities are associated with pregnancy. Several investigators have suggested that there may be a unique procoagulant associated with amniotic tissue and fluid. We identified a cysteine proteinase from malignant tissue, cancer procoagulant, and had reason to believe a similar proteinase may be present in amniotic tissue. Amniotic fluid and extracts of amnion-chorion were purified by immunoaffinity chromatography with an antibody that was developed to cancer procoagulant antigen. The purified amnion-chorion procoagulant initiated clotting in normal and factor VII-deficient citrated human plasma and directly activated pure human factor X in a two-stage clotting assay. It was inhibited by 1 mmol/L of iodoacetamide and 0.1 mmol/L of HgCl2, and the procoagulant activity was activated by 10 mmol/L of KCN; these are classic properties of cysteine proteinases. The pure amnion-chorion procoagulant had the same mol wt and immunologic determinants as cancer procoagulant from rabbit V2 carcinoma, as determined by crossed immunodiffusion, suggesting that the same or very similar proteins were associated with both tissues. Thus, this procoagulant may be derived from both undifferentiated and dedifferentiated cells.