max 41 transgenic mice consistently exhibit elevated numbers of mature granulocytes and monocytes in the peripheral blood and of immature and mature cells of these lineages in the marrow, spleen, lymph nodes and liver. The immature populations are not autonomous and exhibit a normal quantitative responsiveness to proliferative stimulation by the four colony-stimulating factors. The present studies examined three other candidate regulators of granulocyte formation and showed that max 41 cells exhibit normal quantitative responsiveness to stem cell factor, slightly enhanced responsiveness to IL-6 but reduced responsiveness to Flk-ligand. Serum levels of growth factors were not unusually elevated in max 41 mice before or after the injection of endotoxin nor were excessive levels of the four CSFs or IL-6 produced in cultures of max 41 organs. Responses to injected G-CSF were not unusually high in terms of fold-elevations in max 41 mice. Levels of mRNA for various growth factors were not abnormal in max 41 marrow populations although, in crowded cultures, max 41 marrow cells exhibited a higher level of endogenously stimulated colony formation than control cells. max 41 cells also exhibited elevated responsiveness to stimulation by mixtures of growth factors, particularly those in organ-conditioned media. The present observations suggest some possible mechanisms by which a max 41 mouse might achieve a sustained elevation of granulocyte and monocyte production but the data seem insufficient to provide a complete explanation and indicate persisting deficiencies in knowledge of how granulocyte and monocyte production is regulated.