A mutant of recombinant tissue-type plasminogen activator (rt-PA), obtained by deletion of residues Lys296 to Gly302 [rt-PA del(K296- G302)], was previously shown to be resistant to inhibition by plasminogen activator inhibitor-1 (PAI-1) (Madison et al, Nature 339:721, 1989). This mutant was obtained by expression of its cDNA in Chinese hamster ovary cells and purification to homogeneity from conditioned cell culture medium. It was obtained as a single chain molecule with amidolytic activity, specific fibrinolytic activity, and binding to fibrin and lysine, which were comparable or somewhat lower than those of wild-type rt-PA obtained in the same expression system. The plasminogen-activating potential of rt-PA del(K296-G302) in the presence of CNBr-digested fibrinogen was about twofold lower than that of wild-type rt-PA. The inhibition rate of rt-PA del(K296-G302) by recombinant PAI-1 (rPAI-1) was more than 500-fold lower than that of wild-type rt-PA. In a human plasma milieu in vitro, rt-PA del(K296- G302) induced dose-dependent lysis of a 125I-fibrin-labeled plasma clot; equi-effective concentrations (causing 50% clot lysis in 2 hours) were 0.28 micrograms/mL and 0.36 micrograms/mL for mutant and wild-type rt-PA, respectively. In this system, addition of rPAI-1 to the plasma resulted in a concentration-dependent reduction of the fibrinolytic potency of rt-PA del(K296-G302) and of rt-PA; a 50% reduction required 2.4 micrograms/mL and 0.15 micrograms/mL rPAI-1, respectively. Continuous infusion of mutant or wild-type rt-PA over 60 minutes in hamsters with a 125I-labeled plasma clot in the pulmonary artery resulted in dose-dependent clot lysis, with a thrombolytic potency (percent clot lysis per milligram of compound administered per kilogram of body weight) and a specific thrombolytic activity (percent clot lysis per microgram per milliliter steady state rt-PA-related antigen level in plasma) that were not significantly different. Bolus injection in hamsters of 1 mg/kg rPAI-1 followed by bolus injection of 1 mg/kg rt- PA del(K296-G302) or wild-type rt-PA resulted in neutralization of the thrombolytic potency of wild-type rt-PA, while the mutant retained approximately half of its thrombolytic potency. These results indicate that rt-PA del(K296-G302), with a known resistance to inhibition by rPAI-1 in purified systems, maintains this property both in a plasma milieu in vitro and in an experimental animal model of thrombolysis in vivo.(ABSTRACT TRUNCATED AT 400 WORDS).