The pharmacokinetic properties of hPA(B), characterized by the insertion of a urokinase kringle coding region before the double kringle of tPA plus the complete tPA coding region, were investigated and compared to those of melanoma tPA (mtPA). Mean peak plasma concentrations at the end of infusion were 4.7 micrograms/ml for hPA(B) and 4.6 micrograms/ml for mtPA. The pharmacokinetics of both hPA(B) and mtPA showed a biexponential disappearance from plasma which is consistent with a two-compartment model of t 1/2 (lambda 1) = 2 minutes, t 1/2 (lambda 2) = 58 minutes for hPA(B), and t 1/2 (lambda 1) = 2.2 minutes, t 1/2 (lambda 2) = 61 minutes for mtPA. However, this very fast decaying lambda 1 phase of mtPA lasted five times longer than that of hPA(B) which resulted in very low concentrations of mtPA. Thus, hPA(B) exhibited larger AUC, slower clearance rate, and smaller volume of distribution (P less than 0.01) than those of mtPA. The fibrinolytic activity of hPA(B) in rabbit plasma as determined by zymography lasted up to 120 minutes after the end of infusion as compared to that of 2 minutes for mtPA. This indicates that mtPA, despite its t 1/2 (lambda 2) being similar to that of hPA(B), is no longer at physiologically meaningful concentrations at the start of the lambda 2 phase.