Binding of radiolabeled L-cell colony-stimulating factor (CSF) was studied using murine bone marrow and fetal liver cells. With 10(7) cells, saturation of binding was seen with approximately 500,000 cpm of 125I-CSF. Minimal binding was detected after one hour incubation with tracer at 37 degrees C; however, marked cellular uptake of radioactivity was noted after 24-hr exposure to CSF. As judged by autoradiographs, small numbers of myeloblasts, promyelocytes, and large mononuclear cells were labeled with 1-hr exposure to tracer. By 6 hr of incubation, 50%-70% of myeloblasts and promyelocytes and small numbers of late granulocytic cells were labeled. Virtually all myeloblasts and promyelocytes and approximately 50% of myelocytes, metamyelocytes, polymorphonuclear granulocytes, and monocytes were labeled after 24-hr exposure to the radioiodinated CSF. Label was not detected on erythroblasts, eosinophils, or megakaryocytes. Suspensions of fetal liver cells had lower uptake of radioactivity than bone marrow cells. This appeared to result from a lesser concentration of granulocytic cells in fetal liver, as labeling of individual cells was similar with both tissues. In additional experiments, CSF binding to marrow cells was assessed after 30-min exposure to tracer at 0 degrees C. Uptake of 125I-CSF exceeded that observed after 24-hr incubation at 37 degrees C. With this technique, cellular label was also confined to granulocytic and monocytic cells. These findings suggest that purified CSF reacts with and may stimulate immature and mature cells of the granulocytic and monocytic lineages.