Radionuclide imaging for the detection and localization of gastrointestinal bleeding is a highly sensitive and well-established clinical tool. 99Tcm-labelled erythrocytes have permitted the recognition of intermittent bleeding up to 24 h following injection. However, free pertechnetate is actively secreted by gastric mucosa and can result in confounding gut activity. The possibility that pertechnetate may elute from the radiolabelled erythrocytes was investigated in an in vitro model. Blood samples from five subjects undergoing gated cardiac studies were labelled by modified in vivo, modified in vitro and pure in vitro methods and then incubated in serum for 24 h. Automated cell counts and instant thin layer chromatography were used to measure the physical and radiochemical stability of the labelled erythrocytes. No detectable haemolysis occurred and no free pertechnetate was released from the cells. However, an unexpected hydrophilic 99Tcm species became detectable in the supernatant with a quantitative index which increased from an initial value of 0.016 +/- 0.004 to 0.079 +/- 0.012 at 24 h (P less than 0.00001). This index did not differ among the three labelling methods. A chromatographically similar material was identified in the urine of a patient undergoing a gastrointestinal bleed study. The possibility of gut and/or hepatobiliary secretion of this material needs further clarification before delayed positive studies can be relied upon to indicate gastrointestinal bleeding.