Radioiodinated L-cell-derived colony-stimulating factor (CSF) was used to characterize the binding reaction to murine bone marrow cells. The major increment in cell-associated radioactivity occurred over 24 hr incubation at 37 degrees C, but virtually no binding was observed at 4 degrees C. The reaction was saturable with approximately 1 ng/ml of purified CSF. Unlabeled CSF prevented the binding, whereas a number of other hormones and proteins did not compete for CSF uptake. Further specificity studies showed virtually no binding to human bone marrow, which is unresponsive to this form of murine CSF. Minimal CSF uptake was noted with murine peritoneal macrophages, but virtually no binding was detected with thymic, lymph node, liver, or kidney cells. The marrow cell interaction with tracer appeared to require a new protein synthesis, as the binding was prevented by cycloheximide or puromycin. Preincubation of marrow cells in medium devoid of CSF increased the degree of binding after 1 hr exposure to the tracer. This suggests that CSF binding sites may be occupied or perhaps decreased in response to ambient levels of CSF in vivo. Approximately 70% of the bound radioactivity was detected in the cytoplasm at 24 hr. This material was partially degraded as judged by a decrease in molecular weight from approximately 62,000 to 2 peaks of approximately 32,000 and approximately 49,000, but 72% of the binding activity was retained. After plateau binding was achieved, greater than 80% of the radioactivity released into the medium was degraded into biologically inactive peptides with molecular weights less than 10,000. These findings suggest that the interaction of CSF with marrow cells is characterized by binding with subsequent internalization and metabolic degradation into portions of the molecule that are devoid of biologic activity.