One homogeneous population of high-affinity monoclonal antibodies (KD = 0.35 nM) specific for Naja nigricollis toxin alpha has been produced. It neutralizes the biological activity of the toxin under both the vivo and in vitro conditions. The molecular zone of the toxin to which the antibody binds has been precisely defined on the basis of cross-reaction experiments using five derivative of toxin alpha monomodified at a single amino group and two naturally occurring homologous toxins. The epitope is located at the base of the first beta-sheet loop of the toxin, involving the two positive charges at the N-terminal position and lysine-15 proline-18, and probably threonine-16. It is shown that this region is topographically distinct from the "toxic" site of toxin alpha. Several possibilities are offered to explain the mechanisms(s) of specific neutralization.