How might procoagulant activity promote localization of tumor cells in the microvasculature? The image illustrates candidate targets of coagulation proteases generated on the tumor cell. Tissue factor/factor VIIa and factor Xa might activate endothelial PAR2, and thrombin might activate endothelial PAR1 and platelet PAR4 and mediate fibrin formation. These same proteases might act at PARs on the tumor cell itself. Our data emphasize the importance of platelets and fibrinogen as important host effectors of hematogenous metastasis.