Tissue factor (TF) is the primary initiator of the coagulation cascade and is thought to play a key role in the generation of arterial thrombosis. Recent studies have suggested that TF mediates inflammatory processes in the arterial wall and may be an important regulator of intimal hyperplasia. We have employed genetically engineered mice (mTF(-/-) /hTF(+)) with markedly diminished TF activity ( approximately 1% normal levels) o examine the role of TF in mediating the response to arterial injury. mTF(-/-)/hTF(+) displayed a marked reduction in intimal hyperplasia (46% decrease in intimal area, 60% decrease in intimal/medial ratio) in response to femoral artery injury when compared to wild type controls. The decreased intimal hyperplasia seen in low TF mice was noted in a model of vascular injury not associated with significant thrombosis, suggesting that it may be mediated by non-procoagulant properties of TF. Smooth muscle cells from mTF(-/-)/hTF(+) mice grew normally in response to serum, but exhibited a marked defect in cell migration in a modified Boyden chamber assay. In contrast, there was no difference in platelet derived growth factor- induced migration, suggesting that the effect of TF on smooth muscle cell migration is agonist dependent. These data suggest that TF may mediate intimal hyperplasia by regulating smooth muscle cell migration.