Eighteen patients with either clinically or laboratory-detectable gastrointestinal bleeding were studied using 99mTc-pyrophosphate [PYP] in vivo labeled red blood cells. In those patients with massive gastrointestinal bleeding, the site of bleeding was usually detected within the first 1/2 h of scanning. Slower bleeding required longer scanning time. In several patients, bleeding sites were detected which could not be located by other means. As little as 5 ml of ingested whole blood labeled in vivo with 99mTc-PYP could be detected in the upper gastrointestinal tract.