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The thromboplastin activity of T‐241 Lewis sarcoma peripheral fluid and control subcutaneous fluid in C57BL/6Ja male mice was measured, using a calcium reconstitution assay. The material was collected in Millipore filter chambers. At low dilutions of the collected material there was a statistically significant increase in clotting activity of the tumor peripheral fluid. The calcium ion concentration of the collected fluid was also measured. It was also noted that gels were found more frequently, and in larger amounts, in the chambers in tumor‐bearing animals than in the controls.

The thromboplastin activity of tissue culture media, before and afrer incubation with T-241 Lewis sarcoma cells of mouse origin, or with embryonic mouse epithelial or muscle fibroblasts, was mcasured by means of a calcium reconstitution assay. Incubation of the embryonic epithelial or muscle fibroblasts increased the thromboplastin activity of the tissue culture media more than did incubation with tumor cells, even though analysis of protein showed that there were more tumor cells than normal cells present at the time of incubation, It is concluded that there is no evidence for a unique '' Cancer Coagulative Factor '' at least in this system. In vitro studies of the effect of trauma in increasing the release of thromboplastin activity from both normal and tumor tissues, suggest that the mutual destruction of both tissues could account for the elevated levels observed in some patients with cancer.