The blood, spleen and liver of mice were examined by means of electron spin resonance (e.s.r.), throughout the course of myeloid leukaemia induced by intravenous injection of leukaemic spleen cells. In blood, marked increases in the concentrations of iron transferrin and ceruloplasmin occurred within the first 3 days after injection. In the spleen, changes in the concentrations of paramagnetic copper and iron complexes were detectable by about the 5th day, before any measurable splenic enlargement, whilst in the liver changes were detectable by about the 8th day. The changes occurring in blood, spleen and liver during the development of leukaemia appear to be related and they are discussed in terms of iron transport.