The thrombolytic properties of recombinant pro-urokinase (Rec-pro-UK), recombinant active urokinase (Rec-UK) and natural urinary urokinase (Nat-UK) were compared with those of tissue-type plasminogen activator (t-PA) in rabbits with a radiolabeled thrombus in the jugular vein. The thrombolytic agents were infused intravenously over a time period of 4 hr and the extent of thrombolysis measured two hours later. In control animals the extent of thrombolysis was 11 +/- 2% (n = 8) after 6 hr. Nat-UK and Rec-UK had very similar thrombolytic properties. Significant thrombolysis was only obtained with infusion of 240,000 IU per kg (41 +/- 2%, n = 4 for Nat-UK and 37 +/- 4%, n = 4 for Rec-UK) and this was associated with a marked systemic activation of the fibrinolytic system, as evidenced by consumption of plasminogen and alpha 2-antiplasmin and fibrinogen breakdown. Infusion of Rec-pro-UK induced thrombolysis at a dose of 60,000 IU per kg (44 +/- 8%, n = 3) but without associated systemic activation of the fibrinolytic system. In this respect the properties of Rec-pro-UK were similar to those of t-PA, which, however, had a 2- to 4-fold higher specific thrombolytic activity (30,000 IU/kg yielding 48 +/- 1% lysis, n = 4). It is concluded that Rec-UK has very similar thrombolytic properties as Nat-UK and that Rec-pro-UK has a better thrombus-selectivity and less systemic side effects than the active enzymes.