Autoradiographic analysis of the binding of radiolabeled murine IL-6 to murine cells revealed that blast cells, megakaryocytic and monocytic cells in the marrow expressed receptors but not erythroid, eosinophilic or neutrophilic granulocytic cells. In the thymus, spleen and lymph nodes, most cells expressing receptors were single positive CD4+ and CD8+ T lymphocytes while relatively few B220+ slg+ B lymphocytes expressed receptors. The absence of receptors on granulocytes raised a problem with the observed ability of IL-6 to stimulate granulocyte colony formation in mouse bone marrow cultures. Analysis showed that this colony formation was linear with respect to cultured cell numbers and that IL-6 was able to stimulate granulocyte colony formation by at least some transferred isolated clones or individual blast colony cells recultured in the absence of other cell types. Some of the proliferative effects of IL-6 on unfractionated cells may be indirectly mediated, but the data suggest that IL-6 can have direct effects on granulocyte proliferation despite the absence of detectable IL-6 receptors on granulocytic cells.