Abstract The immunological properties of fibrin high affinity urokinase inhibitor complex, a urokinase and PAI (plasminogen activator inhibitor) were measured. The fibrin high affinity urokinase inhibitor complex formed a precipitating arc against antifibrin high affinity urokinase inhibitor complex serum during immunoelectrophoresis. Urokinase-containing plasma also formed an arc in the at region against the same antiserum. Immunoblotting of fibrin high affinity urokinase inhibitor complex demonstrated a band of Mr 100 kDa against antifibrin high affinity urokinase inhibitor complex antibody but failed to demonstrate the same band againt anti-PAI-1 antibody. Urine which had passed through fibrin-Sepharose and urine which was eluted from fibrin-Sepharose both formed bands at Mr 100 kDa, suggesting that urine contains two types of urokinase (two chain) and PAT complexes, which may or may not show fibrin affinity, and that the complex which does show fibrin affinity can be converted to the complex which does not. In plasma, a urokinase and PAI complex (Mr 100 kDa) which was immunologically identical to fibrin high affinity urokinase inhibitor complex was detected by immunoblotting. When urokinase infused into thrombotic patients, such a complex either showed slightly increased intensity or a complex of Mr 200 kDa appeared.