The neuronal alpha7 nicotinic acetylcholine receptor (AChR) binds the neurotoxin alpha-bungarotoxin (alpha-Bgt). Fine mapping of the alpha-Bgt-binding site on the human alpha7 AChR was performed using synthetic peptides covering the entire extracellular domain of the human alpha7 subunit (residues 1-206). Screening of these peptides for (125)I-alpha-Bgt binding resulted in the identification of at least two toxin-binding sites, one at residues 186-197, which exhibited the best (125)I-alpha-Bgt binding, and one at residues 159-165, with weak toxin-binding capacity; these correspond, respectively, to loops C and IV of the agonist-binding site. Toxin binding to the alpha7(186-197) peptide was almost completely inhibited by unlabelled alpha-Bgt or d -tubocurarine. Alanine substitutions within the sequence 186-198 revealed a predominant contribution of aromatic and negatively charged residues to the binding site. This sequence is homologous to the alpha-Bgt binding site of the alpha1 subunit (residues 188-200 in Torpedo AChR). In competition experiments, the soluble peptides alpha7(186-197) and Torpedo alpha1(184-200) inhibited the binding of (125)I-alpha-Bgt to the immobilized alpha7(186-197) peptide, to native Torpedo AChR, and to the extracellular domain of the human alpha1 subunit. These results suggest that the toxin-binding sites of the neuronal alpha7 and muscle-type AChRs bind to identical or overlapping sites on the alpha-Bgt molecule. In support of this, when synthetic alpha-Bgt peptides were tested for binding to the recombinant extracellular domains of the human alpha7 and alpha1 subunits, and to native Torpedo and alpha7 AChR, the results indicated that alpha-Bgt interacts with both neuronal and muscle-type AChRs through its central loop II and C-terminal tail.