Native and density gradient separated rat peritoneal exudate cells were evaluated for their capacity to stimulate granulocyte colony formation and their ability to inhibit colony growth in vitro. Granulocyte colony-stimulating factor (CSF) was primarily elaborated by monocytes and macrophages; neutrophils showed only minimal stimulatory activity. In contrast, freeze-thawed extracts of both monocyte-macrophage and granulocyte fractions were markedly inhibitory to colony growth using a standard leukocyte-derived CSF. Characterization of the inhibitory material showed a low MW, nonlipid, heat-stable substance. In addition to peritoneal leukocytes, other tissues including lymph node, thymic and splenic lymphocytes, liver, kidney and skeletal muscle also contained inhibitory materials. Granulopoiesis, as assessed by in vitro culture techniues, is apparently not influenced solely by a simple feedback loop; rather, leukocytic elements, as well as other tissues, appear to contain and rele ase both stimulatory and inhibitory factors.