The goal of this project is to develop a gamma-emitting agent, suitable for administration to humans, for localizing thrombi or thromboembolic lesions by scintillation camera imaging. A theoretically "ideal" radiopharmaceutical for in vivo nuclear imaging of thrombi and pulmonary emboli would be an easily-labeled molecule which is fibrin-avid but non-adherent to other tissues or circulating proteins, especially fibrinogen. Since fibrin and fibrinogen share many antigenic sites, antibodies to fibrin almost invariably cross-react with fibrinogen, and virtually any cross-reactivity eliminates free antibody from the circulation upon injection. Recently, Kudryk et al developed a monoclonal antibody (Mab) reactive with human fibrin but not cross-reactive with fibrinogen. We have obtained hybridoma clone culture medium and ascites fluid containing this Mab, purified it by immunoaffinity chromatography (IAFC), determined its avidity for dog fibrin, and tested it as an in vivo imaging agent for fresh canine thrombi; in 3 of 3 mongrel dogs, thrombi 1-hour old when labeled Mab was intravenously injected were easily imaged 24 hours later. We intend to study this labeled Mab further in older thrombi and pulmonary emboli, to determine its effectiveness when labeled with I-131 versus Indium-111, and to determine the efficacy of whole labeled Mab versus that of labeled Fab and F(ab')2 fragments. Major problems in radioimmunoimaging of tumors are the relative inaccessibility of circulating radiolabeled tumor-specific antibody to tumor cell-surface antigens, and the small quantity of cell-surface antigen (probably in the picogram or nanogram range). Radioimmunoimaging of thrombi avoids these difficulties: many milligrams of fibrin are involved, and circulating labeled antibody has direct access to an intravascular lesion. Since purification of Mab by IAFC is tedious and slow, we will also purify Mab by preparative isoelectic focusing (PIEF) and compare it with IAFC-purified Mab. PIEF has the potential of greatly accelerating and simplifying Mab purification from ascites fluid. Finally, the best Mab (or Mab fragment) will be used to image thrombi and pulmonary emboli in human subjects.