In order to evaluate the role of the stromal bone marrow microenvironment in regulating granulopoiesis, we have examined the capacity of adult human proximal hemopoietic (PH) and distal nonhemopoietic (DNH) long bone to produce colony-stimulating activity (CSA), characterized the cellular sources of CSA, and quantitated the colony-forming cells (CFU-GM) of marrow from these sites. Stromal elements were obtained from slices of cancellous bone. PH bone marrow stroma contained CFU-GM concentrations similar to aspirated PH marrow and significantly more CFU-GM than DNH bone marrow: 20.7 +/- 4.8/10(5) cells and 25.8 +/- 12.0/mg bone versus 0.81 +/- 0.34/10(5) cells and 0.02 +/- 0.01/mg bone (p less than 0.001). Conditioned media prepared from PH and DNH bone were quantitated for CSA by their ability to promote in vitro granulocyte colony formation of nonadherent human marrow cells. Stromal CSA production was destroyed by freeze--thawing and was radioresistant (4400 rad). Of DNH stromal cells, 15%--30% were monocyte-macrophage, but the slow absolute numbers of these cells suggested alternative CSA cellular sources in distal bones. PH stroma produced significantly more CSA than DNH bone stroma: 0.72 +/- 0.10 versus 0.30 +/- 0.06 U/mg bone (p less than 0.01). The CSA concentration gradient between PH and DNH bones may contribute to the regulation of granulopoiesis in marrow and to the absence of hemopoiesis distally.