The venom of the Eastern green mamba from Africa, Dendroaspis angusticeps, was found to block the binding of 3H-quinuclidinyl benzilate to pure m1 and m4 muscarinic ACh receptors expressed in Chinese hamster ovary cells. The principal toxin in the venom with anti- m1 muscarinic activity was purified by gel filtration and reversed- phase HPLC. This toxin has 64 amino acids, a molecular mass of 7361 Da, and an isoelectric point of 7.04. Its cysteine residues are homologous with those in curare-mimetic alpha-neurotoxins, and with those in fasciculin, which inhibits AChE. At low concentrations the toxin blocked m1 receptors fully and pseudoirreversibly while having no antagonist activity on m2-m5 receptors; the toxin is therefore named “m1-toxin.” At higher concentrations m1-toxin interacted reversibly with m4 receptors, and half of the toxin dissociated in 20 min at 25 degrees C. The affinity of m1-toxin is therefore much higher for m1 than for m4 receptors. By comparison with m1-toxin, pirenzepine has sixfold higher affinity for m1 than for m4 receptors. Autoradiographs of muscarinic receptors in the rat brain demonstrated that m1-toxin blocked the binding of 2 nM 3H-pirenzepine only in regions known to bind m1-specific antibodies. Thus, m1-toxin is a much more selective ligand than pirenzepine for functional and binding studies of m1 muscarinic receptors.