It has been suggested that cancer cell procoagulant activity influences metastasis formation by promoting fibrin deposition around tumors. We have investigated the procoagulant activity of various tumor cell sublines with different metastatic capacity derived from metastatic nodules of a murine fibrosarcoma. All the cells tested possessed a marked thromboplastin-like activity; they were, however, heterogeneous as regards the degree of procoagulant activity; the two cell lines with virtually no metastatic capacity showed 6--8 times higher procoagulant activity than the cells from the parent line; in contrast, the procoagulant activity of the two sublines with higher metastatic capacity did not differ significantly from that of the parent line. These findings support the hypothesis that fibrin is part of a defence reaction against cancer cell invasiveness.