Summary.-The ascorbyl radical concentration has been observed, by means of electron spin resonance spectroscopy, in the blood, spleen and liver of RF/J female mice throughout the development of a myeloid leukaemia. Changes in the concentration of the radical were detectable from an early stage in the disease and did not appear to be directly due to the leukaemic cells but could possibly be due to a reaction against them. Changes in the concentration of the paramagnetic metal ions during the leukaemia have been reported previously and it was found that changes in some of these species correlated with changes in the ascorbyl radical concentration.