The sensitivity of erythropoietic (BFU-E) and granulopoietic (CFU-C) progenitor cells to dexamethasone and cortisone was studied in cultures of mouse bone marrow. Although the log dose-response relationships had a similar form, the BFU-E were much more sensitive than the CFU-C to either glucocorticoid. The dexamethasone concentration for 50% inhibition was 3 X 10)-9) M for BFU-E and 60 X 10(-9) M for CFU-C. The differential sensitivity to cortisone was even greater, with 60% inhibition of BFU-E and 18% inhibition of CFU-C at 0.1 microgram/ml. These findings suggest a specific rather than a general response to glucocorticoids and indicate that granulocyte-macrophage progenitors are less affected than early erythroid progenitors by physiologic concentrations of these hormones.