A simple kit for the preparation of 99mTc-labeled red blood cells in a closed sterile system is described. Whole blood (3-6 ml) is automatically drawn from the patient into a 10-ml Vacutainer tube, containing a freeze-dried stannous citrate formulation, with heparin as an anticoagulant. The kit provides consistent 97% yields in 20 min with small blood samples. The radionuclide is added at the end of the labeling sequence to minimize operator exposure. Freeze-dried kits have performed well after more than 12 months. Some technetium solutions produced low labeling yields. Experimental evidence suggests that the poor yields were due to carrier 99Tc. This effect may explain low labeling yields obtained with some other radiopharmaceuticals.