An adrenal pseudocyst was visualized as a persistent “cold” suprarenal mass on a radionuclide renal angiography study using99mTc-Sn-DTPA. A99mTcpertechnetate blood pool study performed shortly after the renal study again showed an avascular mass, and visualization of vascular abdominal structures was striking. This prolonged and excellent visualization of blood pools is attributable to pertechnetate labeling of red cells from the residual small amounts of circulating Sn (II) derived from the99mTc-Sn-DTPA given shortly before. Experiments confirmed that over 95% of pertechnetate may label red cells when as little as .001 mg Sn per kg of body weight is given (here, in the DTPA preparation) 45 minutes prior to pertechnetate administration. This in vivo labeling of red cells may be turned to advantage in visualizing blood pools during the study of abdominal masses.