Colony-stimulating factor (CSF-1) purified from L-cell-conditioned medium is a haemopoietic growth factor that specifically stimulates the proliferation and differentiation of mononuclear phagocytes. Using radioactively labelled CSF-1 [( 125I]CSF-1), the presence of specific CSF-1 receptor has been identified in the cells of the mononuclear phagocytic series and their precursors only. To determine the fate of [125I]CSF-1 bound to peritoneal exudate macrophages (PEM) at 37 degrees C, we have examined the distribution of radioactivity as a function of time by quantitative electron microscopic autoradiography. At 0 degrees C, we have localized the initial step in the binding of [125I]CSF-1 to the plasma membrane and its invaginations of the mouse PEM. Approximately 16% of the macrophages were not labelled at this time point. When the temperature was raised to 37 degrees C, the labelled CSF-1 was internalized progressively by the cells in a time-dependent fashion. The proportion of grains associated with the phagolysosome compartment increased progressively, reaching a plateau by 40 min after warming up, while the relative areas of the surface membrane and its invaginations decreased in invaginated membrane. At 37 degrees C, incubation with unlabelled CSF-1 resulted in a "down-regulation' of the subsequent [125I]CSF-1-binding activity by PEM in a time- and dose-dependent fashion. The restoration of CSF-1-binding activity after CSF-1 induced down-regulation was inhibited by cycloheximide, a potent protein synthesis inhibitor. These data provide direct evidence that at 37 degrees C, saturable binding of CSF-1 to PEM is followed by internalization and cellular degradation of the ligand and possibly its receptor by phagolysosomes.