[en] Indium-111-labeled monoclonal antibody 64C5 specific for the beta-chain of fibrin monomer was used to image canine (n = 6) experimental pulmonary emboli (at least one barium-thrombin and one copper-coil induced clot per dog). Uptake of ¹¹¹In-64C5 and 125I-control-DIG26-11 were compared in 10 clots (7 barium-thrombin and 3 copper-coil) identified in the lungs. There was no difference in the blood clearance of ¹¹¹In-64C5 and 125I-DIG26-11. Uptake of ¹¹¹In-64C5 (0.183 ± 0.105, mean %ID/g) was greater than 125I-DIG26-11 (0.024 ± 0.025) in pulmonary clots (p less than 0.001). Mean thrombus to blood ratios at 24 hr were 6.78:1 for 64C5 and 0.57:1 for DIG26-11. The clots visualized in vivo were larger (0.315 ± 0.381 g) than clots not visualized (0.089 ± 0.098). Negative images were recorded in three dogs with pulmonary emboli, injected with ¹¹¹In-labeled control monoclonal antibody 3H3. These data suggest that ¹¹¹In-labeled antifibrin can detect large pulmonary emboli in vivo