To determine the effectiveness of abdominal imaging with RBCs labeled in vivo with Tc-99m, for the detection of gastrointestinal (GI) bleeding, 28 control subjects and ten patients with suspected bleeding underwent scintigraphy at 0-24 hr after tracer injection. Colonic activity was noted in one of the controls within 3 hr of injection, and in five of ten controls at 24 hr, all of whom had initial gastric activity. Of the ten patients with suspected GI bleeding, eight had documented active bleeding; seven of these had positive scintigrams. Nasogastric (NG) suction markedly decreased the presence of initial gastric activity in the patients with active bleeding. With this blood-pool radiopharmaceutical, frequent imaging of the abdomen over 24 hr can be done to test active bleeding. Continuous NG suction is recommended to reduce accumulation of gastric activity. These results suggest that red blood cells labeled in vivo with Tc-99m provide a sensitive method of detecting active GI bleeding.