We studied toxic effects of various schemes of cytostatic treatment on central and peripheral pools of hemopoietic precursor cells in patients with III-IV stages of lung cancer and breast cancer. It was found that erythro- and granulomonocytopoietic precursors are characterized by high resistance to cytostatic treatment compared to morphologically discernible bone marrow elements, which is probably a evolutionary developed property of this cell type aimed at rapid recovery of the hemopoietic tissue.