Intravenously injected 99mTc-DTPA was evaluated in 64 patients for its efficiency in detecting and localizing sites of acute upper and lower gastrointestinal (G.I.) bleeding. These studies were correlated with endoscopic and surgical findings. There were 34 bleeders and 30 non bleeders giving a sensitivity of 90%, specificity of 82% and accuracy of 86%. Of these, 49 were upper G.I. studies (stomach 21 and duodenum 28) and 15 were lower G.I. studies (small intestine 8, large bowel 7). Of the 49 upper G.I. studies, 27 showed active bleeding while 22 showed no bleeding at the time of the study resulting in a sensitivity of 87.5%, specificity of 76% and accuracy of 82%. Of the 15 lower G.I. studies, 7 were bleeders while 8 were non bleeders. All the lower G.I. bleeding sites were accurately localized with the 99mTc-DTPA. An incidental finding of these studies was the localization of 99mTc-DTPA in the site of inflammatory and malignant lesions of the G.I. tract. Of the 64 studies, 18 inflammatory and malignant lesions were detected with the IV injected 99mTc-DTPA; 10 were bleeders while 8 were non bleeders. Image subtraction of early from delayed images was helpful to differentiate bleeding from non bleeding cases in this last group of studies.