Radioiodinated granulocyte-macrophage colony-stimulating factor (125I-GM-CSF) binds to specific receptors (molecular weight approximately 50,000 daltons) on the murine myelomonocytic leukemia, WEHI-3BD+. At 4 degrees C 125I-GM-CSF remains on the surface of the cells and can be eluted by washing the cells with acidified isotonic buffer. When the cells are warmed to 37 degrees C, the 125I-GM-CSF is internalized rapidly (t 1/2: 7 min). The internalisation appears to be entirely receptor mediated and is independent of energy sources inhibited by sodium azide. This GM-CSF-mediated internalisation is not due to a general increase in the turnover of cell surface molecules as the specific binding of 125I-transferrin is not affected by incubation of WEHI-3BD+ cells with GM-CSF. The initial 125I released when the cells are warmed to 37 degrees C appears to be intact 125I-GM-CSF; however, after 2 h 80% of the 125I released was not precipitable with trichloroacetic acid and presumably represented degraded 125I-GM-CSF. Ammonium chloride or monensin reduced the release of 125I-GM-CSF from the cells, suggesting that the receptor-bound ligand was processed through the lysosomes. A considerable proportion of the internalised GM-CSF receptors were recycled to the surface and were available for ligand binding. Synthesis of new GM-CSF receptors contributed to the re-expression of GM-CSF receptors after down-regulation and it is possible that the GM-CSF enhances the synthesis of its own receptors.