Bone marrow, spleen and liver of healthy and erythroleukemic mice have been studied by autoradiography. A peripheral zone, where the most of cells were involved in mitosis, and a central one, where the mitotic index was lower, were established in the bone marrow. Normally a zone of high proliferative activity was found in the subcapsular region of the spleen. Boundaries of such zones in the bone marrow and spleen of erythroleukemic mice disappeared. Most of the cells forming the leukemia infiltrates in the liver are in the phase of DNA-synthesis.