Adherent stromal cells from mouse bone marrow inhibited the formation of granulocyte/monocyte (G/M) colonies induced in vitro by colony-stimulating factor (CSF). This inhibition occurred both when crude conditioned media obtained from various sources were used to induce colony formation or when a pure CSF preparation from mouse lung origin was tested. The inhibition did not appear to be toxic in nature since despite the lack of colony formation, progenitor CFU-C proliferated in the presence of stromal cells. Medium conditioned by adherent stromal cells was devoid of inhibitory activity when incorporated into the culture medium used for G/M colony formation, indicating that the inhibitory activity may not be present in a soluble form. Inhibitors of prostaglandins did not affect G/M colony formation. In contrast, D-glucose and a number of other free monosaccharides but not pyruvate lactate or glycerol induced formation of myeloid colonies in the presence of stromal cells. This did not require addition of exogenous CSF. Released factors concentrated from serum-free medium conditioned by stromal cells exhibited colony-stimulating activity provided that the medium contained a high glucose concentration during incubation. It is proposed that stromal cells produce a resident CSF that, in contrast to exogenous CSF species, is capable of inducing myelopoiesis within the bone and marrow stroma.