The effect of inhibition of factor XIIIa with 2-(l-acetonylthio)-5-methylthiazolo[2,3-b]1,3,4-thiadiazolium perchlorate (L-722,151) on coronary thrombolysis and reocclusion was studied in an acute dog model of electrically induced coronary thrombosis. L-722, 151 (0.1 mg/kg/min intravenously [IV] or placebo was administered 15 minutes before current initiation (150 μA) and for the duration of the experiment (270 minutes). Fifteen minutes after thrombus formation, heparin (300 U/kg, IV) was administered, followed 45 minutes later by recombinant tissue-type plasminogen activator (tPA) (10 μg/kg/min, IV for 90 minutes). Placebo-treated animals thrombosed at 48.9 ±8.1 minutes (mean ± SEM) and reperfused in response to tPA at 49.1 ± 9.3 minutes. L-722, 151 pretreated animals thrombosed at 44.4 ± 9.7 minutes and reperfused in response to tPA at 16.4 ± 2.8 minutes (P < .05 v vehicle). Furthermore, residual thrombus mass was reduced by L-722, 151 from 6.9 ± 1.9 mg in placebo-treated animals to 1.7 ± 0.6 mg (P < .05 v vehicle). Acute reocclusion occurred in 86% of placebo and in 75% of L-722, 151-treated animals. The incidence of tPA-induced reperfusion in L-722, 151-treated dogs was 100% (8 of 8), whereas only 70% (7 of 10) of placebo-treated dogs reperfused. These results demonstrate that pretreatment with L-722, 151 hastens reperfusion time threefold and reduces residual thrombus mass. These effects occurred with no change in systemic blood pressure in response to L-722,151. When L-722,151 was administered 15 minutes after thrombus formation in a separate group of dogs (n = 5), no beneficial effect on thrombolysis time or thrombus mass was observed. Thus, the specific factor XIIIa catalyzed crosslinking reaction(s), which may determine(s) resistance to plasmin-mediated fibrin degradation, occur(s) rapidly. Inhibition of this cross-linking by pretreatment with L-722,151 promotes tPA-induced thrombolysis.