Results of three different regimens of extracorporeal irradiation of the blood (ECIB) are compared (1 X 20,2 X 10, and 4 X 5hr) in a rat model for human acute myelocytic leukemia. The D0 for X rays of leukemic cells in the blood was 68 cGy. The transit dose varied from 380 to 660 cGy. Evaluation was performed by follow-up of the number of peripheral leukemic cells (and lymphocytes) and quantitative cell compartment analysis by measuring the distribution of infused 51Cr-labeled leukemic cells. Repeated sessions of ECIB (2 X 10 hr; 4 X 5 hr) proved far more effective in terms of cell depletion than one long session (1 X 20 hr). Chromium studies indicated that the sizes of the blood compartments and of the rapidly exchangeable tissue pool are markedly reduced (maximally by about 70%). The more-sessile organ compartments are only slightly influenced. It appeared that the traffic of leukemic cells is subject to a regulatory mechanism. The more the peripheral compartment is depleted, the more cells are mobilized from the rapidly and slowly exchangeable tissue compartments.