Extracellular ATP induces cation fluxes in thioglycolate-elicited mouse peritoneal macrophages and the J774 macrophage cell line apparently due to ligation of a plasma membrane receptor for ATP4-. We report that ATP permeabilizes the plasma membrane of J774 cells to 6-carboxyfluorescein (376 Da), lucifer yellow (457 Da), and fura-2 (831 Da) but not to trypan blue (961 Da), Evans blue (961 Da), or larger dye conjugates. We employed fluorescence microscopy and quantitative fluorimetry to study entry of lucifer yellow into the cytoplasm of J774 cells. Permeabilization to lucifer yellow appears to be mediated by the same ATP4- receptor that induces cation fluxes because it was inhibited by divalent cations and low pH, was mediated by the nonhydrolyzable analog adenosine 5'-(beta, gamma-imido)triphosphate, and because a variant J774 cell line resistant to ATP-induced Rb+ efflux did not take up lucifer yellow when exposed to ATP. ATP permeabilization was reversed within 5 min by removal of ATP or by addition of divalent cations. ATP also caused a transient increase in lucifer yellow uptake by pinocytosis. These data suggest that ATP4- ligates a receptor on macrophages which induces the formation of a channel admitting molecules less than or equal to 831 daltons into the cytoplasmic matrix and that removal of ATP4- from the medium causes rapid channel closure.