Proliferation activity of bone marrow cells was studied in six patients with acute myeloid leukemia treated by extracorporeal irradiation of the blood (ECIB). One patient with acute lymphoblastic leukemia in full remission was also studied and one patient received splenic irradiation in addition to ECIB. The observations support the hypothesis that ECIB may alter feedback mechanisms of growth control within the leukemic tissue, resulting in an increased proliferative fraction. It is suggested that the nonproliferative leukemic blast cell is not an end cell but is capable of being recalled into cell cycle by an appropriate stimulus such as ECIB.

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