The initial histological changes of leukemia were investigated in rats to which 1‐ethyl‐ 1‐nitrosourea and 1 butyl 1 nitrosourea were orally administered. The appearance of orthochromatic erythroblasts in the peripheral blood was used as the index of the initial stage of leukemia. The rat leukemia progressed from solitary lesions to scattered and further diffuse lesions. These leukemias are thought to begin as one, or only a few nodular foci, mainly in the bone marrow and partly in the spleen. Acta Pathol Jpn 42: 158–16
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