The clot uptake of labeled active and inhibited t-PA was compared. The most efficient inhibition was obtained with diisopropyl fluorophosphate (DFP) after 4 h incubation at room temperature. Enzyme activity was followed by fibrin-plate assay, radioactivity-release technique and proton magnetic resonance (PMR). The novel PMR method developed by us is sensitive to the effect of as low as nanogram amounts of t-PA on the interaction between the fibrin and the compartmentalized water trapped in the clot. Binding of labeled enzyme to fibrin-coated plates showed that the deactivation by DFP did not impair the affinity of t-PA for fibrin. A rapid binding of 125I-labeled t-PA to the clot occurred, which reached a maximum in 30 min and declined with time. This pattern was explained by consecutive clot binding and lysis. The binding of DFP-t-PA to the clot differed markedly from that of the active protein; 2 h post-incubation the uptake of DFP-t-PA was more than double that of the untreated t-PA. Parallel measurements in clots prepared from human blood showed a qualitatively similar trend. The biodistribution of radiolabeled t-PA in mice was similar for the active and inhibited forms. Blood activity reached 10% of the injected dose within 10 min. DFP-t-PA may prove to be a useful reagent for in-vivo localization of thrombi.