1. Erythrocyte superoxide dismutase activity was determined in 46 adults, 10 paediatric patients without anaemia and 11 paediatric patients with several types of anaemia. No difference was found in superoxide dismutase activity between adults and normal children. 2. The use of multiple parameters appeared to be important for the evaluation of erythrocyte superoxide dismutase activity. 3. Erythrocyte superoxide dismutase activity in three cases of typical Fanconi's anaemia was markedly depressed, when expressed either per mg of haemoglobin or per number of cells, and its aetiological significance is discussed.