Technetium-99m-labelled red blood cell scintigraphy was used in the investigation of 41 patients with major rectal haemorrhage. Red cells were labelled in vivo with technetium-99m pertechnetate and serial scans (0-36 h) were taken using a large field gamma camera. Twenty-two men and 15 women, mean age 71.3 years (range 32-91 years), and four children were studied. Forty-three scans were performed in all and there were 31 positive and 12 negative results. In each case the accuracy of the scan in localizing the bleeding site was checked either by independent investigations or at laparotomy. A definite bleeding site was identified in 30 cases and red cell scintigraphy correctly localized 29 of these (sensitivity 97 per cent). In the remaining 13 cases a bleeding site was not identified by any of the independent investigations and in this group there were two false positive scans (specificity 85 per cent). The investigation had a high predictive value when both positive (94 per cent) and negative (92 per cent). These data suggest that technetium-99m-labelled red cell scintigraphy should be used more widely in the investigation of patients with suspected lower gastrointestinal haemorrhage.