Protective antigen (PA), a component of anthrax toxin, mediates translocation of the toxin's lethal and edema factors (LF and EF, respectively) to the cytoplasm, via a pathway involving their release from an acidic intracellular compartment. PA63, a 63-kDa proteolytic fragment of PA, can be induced to form ionconductive channels in the plasma membrane of mammalian cells by acidification of the medium. These channels are believed to be comprised of dodecyl sulfate-resistant oligomers (heptameric rings) of PA63 seen by electron microscopy of the purified protein. Here we report that the PA63-mediated efflux of 86Rb+ from preloaded CHO-K1 cells under acidic conditions is strongly inhibited (> or = 70%) by LF or LFN, a PA-binding fragment of LF. Control proteins caused no inhibition. Evidence is presented that the inhibition involves partial blockage of ion conductance by the PA63 channel. Also, oligomer formation is slowed somewhat by LF at pH values near the pH threshold of channel formation (pH approximately 5.3), suggesting that channel formation may also be retarded under these conditions. The relevance of these results to the location of the LF-binding site on PA63 and the mechanism of LF and EF translocation is discussed.