A new method of labelling human RBCs with 99mTc by the use of Sn-alpha-D-glucose 1-phosphate (GP) is presented. It was tested for spleen imaging in 16 normal volunteers. The labelling was carried out during the heating (30 min at 49.5 degrees C) to damage the cells and cooling (10 min at R.T.) steps. The labelling efficiency was 95.5 +/- 1.5% with Sn-GP and was better than Sn-PYP (61.5 +/- 25.0%). The radioactivity retention of RBCs was greater than 96% after 6 washings. The spleen was delineated very well in all the subjects. The spleen activity reached a plateau at 20 min post-injection. Spleen-to-liver and spleen-to-cardiac blood pool concentration ratios were high (10.4 +/- 2.2 and 10.1 +/- 3.2, respectively) calculated at 30 min. The method is simple, rapid and efficient.