IL-6 was found to be produced by bone marrow cell fraction enriched for hemopoietic stem cells in response to IL-3 stimulation. To confirm the production of IL-6 by hemopoietic stem cells, we studied the response of W/Wv mouse bone marrow cells to IL-3 stimulation in terms of IL-6 production. Genetically anemic mice of W/Wv genotype are known to be depleted of normal hemopoietic stem cells. Bone marrow cells of W/Wv mice produce IL-6 without IL-3 stimulation. But bone marrow cells of the litter mate (+/+) mice studied as controls do not produce IL-6 without stimulation. IL-3 induced the production of IL-6, GM-CSF and G-CSF from the bone marrow cells of +/+ mice. Gene expression of IL-6, GM-CSF, G-CSF and IL-3 in these mice was detected in total bone marrow cells after IL-3 stimulation. In contrast, bone marrow cells of C3H mice did not produce IL-3 even after IL-3 stimulation. Bone marrow cells of C3H mice reportedly produce a diffusible IL-3 inhibitor (NIL-3), but those of C57BL mice do not. Present findings indicate the strain-dependent differences in hemopoietic stimulating factor production. The constitutive IL-6 production in W/Wv mice might be due to a constant and strong demand for blood cell production of the anemic mice.