Neoplastic cells produce procoagulants responsible for hypercoagulation states frequently observed in cancer patients. It is accepted that two major procoagulants from malignant tissue are tissue factor (TF) and a direct activator of coagulation factor X called cancer procoagulant (CP). Direct factor X-activating activity of cultured human malignant melanoma WM 115 cells has been analyzed in the cell extracts, whole cells and in the medium after the cell culture. The factor X-activating activity was detected in the malignant cell lysates but not in the cultured medium or intact malignant cells. The lysates contained no TF as determined by Western blotting and enzyme-linked immunosorbent assay (ELISA) using anti-TF monoclonal antibody. The enzymatic characteristics of the activity was typical for CP. The results suggest that cancer procoagulant is an intracellular protein.