Incubation of murine bone marrow-derived macrophages (BMM) in medium containing recombinant macrophage colony-stimulating factor (rM-CSF) stimulated influx, efflux, and the net accumulation of the fluid-phase pinocytic marker, lucifer yellow (LY). Stimulation was dose dependent, occurred within 5 min of addition of the growth factor, and was sustained. Previous experiments had shown that BMM treated with PMA were stimulated to accumulate LY, but compared with rM-CSF-treated cells, the onset of stimulation in PMA-treated macrophages was slower. In further comparisons of rM-CSF- and PMA-stimulated LY accumulation, it was found that rM-CSF-stimulated pinocytosis could be abolished by pretreatment with 0.5 mg/ml trypsin, whereas neither unstimulated nor PMA-stimulated LY accumulation was affected by trypsin pretreatment. These findings indicate that the rM-CSF response was initiated at the cell surface, while the PMA response occurred via intracellular (or trypsin-resistant) receptors. However, once initiated, the pinocytic responses elicited by either agent were very similar. First, rM-CSF-treated cells, like PMA-treated cells, showed extensive ruffling and formation of large phase-bright pinosomes. Second, both rM-CSF- and PMA-stimulated LY accumulation could be inhibited by treatment of cells with the cytoskeleton destabilizing drugs nocodazole, colchicine, or cytochalasin D. Finally, rM-CSF, like PMA, was found to stimulate efflux of LY from cells preloaded with the dye. Thus, both rM-CSF and PMA stimulate the net rate of solute flow through the macrophage endocytic compartment.