Autologous red blood cells can be labelled in vitro with several radionuclides, including Cr-51, In-111 and Tc-99m, or in vivo with radio-iron. Cr-51 has a half-life of sufficient duration to enable measurement of red cell lifespan, but this is now largely of historical interest. Tc-99m-labelled red cells are used to localise sites of bleeding, especially in the intestinal tract, measure left ventricular ejection fraction, and image haemangiomata. Imaging occult intestinal bleeding is a major challenge for departments of nuclear medicine. Red cells damaged by gentle heating in vitro and then labelled with Tc-99m accumulate in the spleen because of loss of deformability and are therefore used to distinguish splenunculi from suspected intra-abdominal deposits.