The administration of a radiolabelled monoclonal antibody against tissue plasminogen activator allows detection of areas with increased fibrinolytic activity, i.e. those with an active thrombotic lesion. Eight patients with phlebographically verified deep venous thrombosis were examined. At the time of immunoscintigraphy study they were examined receiving anticoagulant therapy. Some 75-85 MBq indium 111-labelled antibody were injected, and scintigrams were obtained after 30 min and after 24 h. The precise site of the thrombus could not be visualized after 30 min due to high background activity, whereas after 24 h it was detectable in all patients. The thrombus/background ratios achieved are twice as high as those observed in a human antifibrin antibody study. These preliminary data suggest a high sensitivity of our t-PA-specific antibody for the detection of active deep venous thrombosis in man, and our antibody seems to offer theoretical advantages over both platelet and fibrin-specific antibodies.