Stromal cells are known to produce granulocyte colony-stimulating factor (G-CSF) and interleukin 6 (IL-6). To assess the function of bone marrow stromal cells in the pathological state, we studied their in vitro ability to produce G-CSF and IL-6 in 9 patients with aplastic anemia and 5 normal volunteers, using an enzyme immunoassay method. Constitutive production of IL-6, but not G-CSF, was detected in almost all cases. Interestingly, the inducible production of G-CSF and IL-6 by stromal cells after stimulation with interleukin-1 and/or lipopolysaccharides was severely depressed in 3 patients with aplastic anemia. Our results may reflect poor hematopoietic activity in the bone marrow in some cases of aplastic anemia.