AbstractThe 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.