Tissue factor (TF) is a transmembrane glycoprotein that initiates coagulation and plays a critical role in regulating hemostasis and thrombosis. We have recently reported a naturally occurring, soluble form of human tissue factor (asTF) generated by alternative splicing. This splice variant has a novel C-terminus with no homology to that of the full-length TF (flTF), lacks a transmembrane domain, and is active in the presence of phospholipids. Mouse models offer unique opportunities to examine the relative importance of flTF and asTF in mediating thrombosis, the response to arterial injury, and ischemic damage. To that end, we have identified and characterized murine asTF (masTF). Like the human splice variant, masTF lacks a transmembrane domain and has a unique C-terminus. We have generated antibodies specific to masTF and murine flTF (mflTF) to examine the expression of both forms of TF. masTF antigen is widely and abundantly expressed, with a pattern similar to that of mflTF, in adult tissues, in experimentally induced thrombi, and during development. These studies demonstrate that masTF contributes to the pool of total TF and may thus play an important role in mediating TF-dependent processes.