Description: OBJECTIVE To measure thrombin generation by high and low tissue factor (TF)-expressing canine cancer cell lines. SAMPLE Canine cell lines CMT25 (highTF-expressing mammary gland tumor cell line) and HMPOS (lowTF-expressing osteosarcoma cell line). PROCEDURES Thrombin generation by cancer cells was measured in pooled normal canine plasma by use of calibrated automated thrombography without added trigger reagents. Results were expressed as lag time, time to peak thrombin concentration, peak thrombin concentration, and total thrombin concentration or thrombin generation potential. Corn trypsin inhibitor, hirudin, and annexin V were used to inhibit contact activation, thrombin formation, and phosphatidylserine activity, respectively. Pooled normal human plasma deficient in coagulation factors VII,VIII, IX, X, XI, or XII was used to assess the role of individual coagulation factors on thrombin generation. RESULTS CMT25 generated significantly more thrombin than did HMPOS (mean с SD, 3,555 с 604nM thrombin min and 636 с 440nM thrombin min, respectively). Thrombin generation of CMT25 was dependent on factor VII and phosphatidylserine and was independent of contact activation. In contrast, thrombin generation of HMPOS was attributed to contact activation. CONCLUSIONS AND CLINICAL RELEVANCE High TF-expressing canine mammary cancer cells generated thrombin in a plasma milieu in vitro in a factorVII- and phosphatidylsehne-dependent manner.These findings support a role forTF in hypercoagulability detected in dogs with mammary gland tumors and potentially for other tumors that strongly express TF