Vascular-targeted scFv/uPA-T provides “on-demand” prophylactic fibrinolysis triggered by thrombin. Lysis of pulmonary clots initiated by injection of thromboplastin in mice 0.5 hour or 3 hours after a bolus injection of equal molar amounts (5 nmol) of scFv/uPA-T, scFv/uPA, and lmw-scuPA (n = 3–4; *P < .05, scFv/uPA-T vs lmw-scuPA; #P < .05, scFv/uPA-T vs scFv/uPA). Error bars represent SEM.