Background - Plasmin dependent thrombolytic agents are potentially pro-thrombotic and pro-inflammatory. Alfimeprase, a zinc containing metalloproteinase, degrades fibrin directly and achieves thrombolysis independent of plasmin formation. This study examines the hypothesis that thrombolysis in the absence of plasmin generation results in improved myocardial salvage upon reperfusion. Methods and Results - The thrombolytic effects of rt-PA (0.022mg/kg, 1/10 of which was administered as a loading dose and the rest 9/10 was infused over 60 min by intra-coronary (i.c.) administration) or alfimeprase (0.5 mg/kg over 1 min i.c.) were evaluated in a canine model of arterial thrombosis involving electrolytic injury of the left circumflex (LCX) coronary artery. Both agents induced thrombolysis, onset of reperfusion being more rapid after alfimeprase compared to rt-PA (1.5±0.6 vs. 10.1±2.1 min). In the absence of adjunctive therapy, time to reocclusion after alfimeprase was 3.2±0.5 min, compared to rt-PA 77.5±31.9 min. The GPIIb/IIIa platelet receptor antagonist, CRL42796, prolonged reperfusion time after thrombolysis with alfimeprase or rt-PA. The effect of each lytic agent on myocardial infarct size was examined in a separate group of dogs subjected to 60 min of LCX coronary artery ligation and 4 hrs of reperfusion. Myocardial infarct size, expressed as percent of the risk region, was larger (32.16±3.95%) after rt-PA compared to alfimeprase (19.85±3.61%) or that of the saline control group (18.46±3.34%). Conclusions - rt-PA in contrast to alfimeprase, a direct acting fibrinolytic agent, is associated with an increase in myocyte reperfusion injury.