m1-Toxin was found to slow the dissociation of [3H]N-methyl-scopolamine (NMS) and [3H]pirenzepine from m1 muscarinic receptors expressed in the membranes of Chinese hamster ovary cells. When toxin-NMS-receptor complexes were formed in membranes and then dissolved in digitonin, or when these complexes were formed in solution, the toxin completely stopped the dissociation of [3H]NMS for 6 hr at 25 degrees C. Toxin-receptor complexes formed in membranes or in solution were also highly stable in solution at 25 degrees, as shown by the ability of the toxin to prevent the binding of [3H]quinuclidinyl benzilate (QNB). [3H] QNB-receptor complexes were equally stable, whereas unliganded soluble receptors lost most of their ability to bind QNB within an hour. Toxin-receptor complexes could be partially dissociated by incubation at 37 degrees in the presence of digitonin and [3H]QNB, and the freed receptors were then labeled. These results demonstrate that m1-toxin binds allosterically and pseudoirreversibly to m1 receptors, and that the toxin can stabilize the outward-facing pocket of m1 receptors which contains and binds competitive antagonists. The allosteric nature of the binding of m1-toxin should prove to be useful for such unusual purposes as stabilizing the binding of readily reversible and/or nonselective ligands specifically to m1 receptors, for purifying labeled or unlabeled receptors by affinity techniques which recognize the toxin, for recognizing receptors with genetically or biochemically altered primary binding sites, and for stabilization of the native conformation of m1 receptors for structural studies.