In response to tetra-anionic ATP4-, P2Z receptors signal opening of a non-selective plasma membrane pore which permits passage across cell membranes of ions, nucleotides and other small molecules that are usually membrane impermeant. P2Z receptor-induced pores on murine macrophages, macrophage-like cell lines and human culture-matured macrophages are permeable to molecules of up to 831 Da. The function of P2Z receptors is unknown. Also unknown is whether the binding site for ATP4- and the transmembrane pore, the properties that characterize P2Z receptors, reside on a single protein or reflect the activities of two or more proteins. That ATP(4-) -unresponsive cell lines do not express connexin 43 has led Beyer and Steinberg to suggest that opening or surface expression of this gap junction protein is induced by P2Z receptors. Xenopus oocytes, injected with cRNA transcribed from a pool of 100 cDNA clones isolated from a murine macrophage-derived cDNA library, and treated with ATP4-, express a non-selective membrane conductance characteristic of P2Z receptors. The conductance induced with cRNA is smaller than that induced by mRNA from macrophages, suggesting the presence of a dominant subunit of a multicomponent receptor in this pool of 100 cDNA clones.