We investigated the effects of recombinant human granulocyte-macrophage colony-stimulating factor (rhGM-CSF) and recombinant human granulocyte colony-stimulating factor (rhG-CSF) therapy on the natural killer (NK) cell lineage in patients with aplastic anemia and myelodysplastic syndrome. Selected bone marrow (BM) cells were prepared by the elimination of nylon wool-adherent cells and mature T and NK cells from BM cells. The frequency of BM NK progenitors relative to BM cells selected was significantly decreased 4 weeks after the start of rhGM- CSF therapy (P less than .01), while the peripheral blood NK cell count and NK activity were also significantly decreased (P less than .05). A return to the pretreatment levels was seen 4 weeks after the cessation of treatment in all cases. No suppressive effect was noted in the patients who received rhG-CSF therapy. These results suggest that rhGM- CSF therapy suppresses the generation of NK cells from human BM NK progenitors.