L‐cell colony‐stimulating factor (CSF‐1) is a sialoglycoprotein of molecular weight 70,000 daltons that specifically stimulates macrophage colony formation by single committed cells from normal mouse bone marrow and by various classes of more differentiated tissue‐derived mononuclear phagocyte colony‐forming cells (Stanley et al., 1978). CSF‐1 interacts with target cells by direct and specific binding to membrane receptors (CSF‐1 receptors) that are present only on cells of the mononuclear phagocyte series and their precursors. We studied the effect of tumor‐promoting phorbol esters on the binding of 125I‐labeled CSF‐1 (125‐CSF‐1) to murine peritoneal exudate macrophages (PEM). Biologically active TPA (12‐O‐tetradecanoyl phorbol‐13‐acetate) inhibits the binding of 125I‐CSF‐1 to its receptor on PEM. This inhibition exhibits temperature, time, and concentration dependence. At 37°C, maximum inhibition occurred at about 10−7 M; inhibition was 50% at 5 × 10−9 M. At 0°C, the inhibitory activity of TPA is diminished. The action of TPA on PEM is transient. Treated cells recover their 125I‐CSF‐1‐binding activity whether TPA is later removed or not. The process of recovering CSF‐1‐binding activity is completely blocked by the addition of cycloheximide. When several phorbol derivatives were tested for their inhibitory activities, only biologically active phorbol esters were found to possess such activities. Furthermore, the inhibitory activities of various phorbol esters are proportional to their tumor‐promoting activities. Inhibition appears to be due to a reduction in the total number of available CSF‐1 receptors rather than a decrease in receptor affinity.