Localization of colorectal bleeding using a gamma camera was attempted in a prospective study of 24 patients after in-vivo labelling of their erythrocytes with 99mTc-pertechnetate. In 7 of 8 patients with bright-red rectal bleeding a correct localization was given by scintiphotos, thus helping the surgeon to plan the operative procedure. Of 11 patients with recently acute colorectal bleeding but no signs of actual haemorrhage, 8 had no scintigraphic bleeding. Five investigations had to be excluded due to technical errors. The scintigraphic classification was correct in a significant number of patients (p less than 0.02). It is concluded that bleeding can be identified in colorectum at brisk-haemorrhagic stools and be correctly localized to the right or left side of the colon. This non-invasive scintigraphic procedure should therefore be undertaken before an abdominal 2-3 vessel arteriography is performed.