A novel "weak toxin" (WTX) from Naja kaouthia snake venom competes with [125I]α-bungarotoxin for binding to the membrane-bound Torpedo californica acetylcholine receptor (AChR), with an IC50of ∼2.2 μM. In this respect, it is ∼300 times less potent than neurotoxin II from Naja oxiana and α-cobratoxin from N. kaouthia, representing short-type and long-type α-neurotoxins, respectively. WTX and α-cobratoxin displaced [125I]α -bungarotoxin from the Escherichia coli-expressed fusion protein containing the rat α7 AChR N-terminal domain 1-208 preceded by glutathione S-transferase with IC50 values of 4.3 and 9.1 μM, respectively, whereas for neurotoxin II the IC50 value was 100 μM. Micromolar concentrations of WTX inhibited acetylcholine-activated currents in Xenopus oocyte-expressed rat muscle AChR and human and rat α7 AChRs, inhibiting the latter most efficiently (IC50of ∼8.3 μM). Thus, a virtually nontoxic "three-fingered" protein WTX, although differing from α-neurotoxins by an additional disulfide in the N-terminal loop, can be classified as a weak α-neurotoxin. It differs from the short chain α-neurotoxins, which potently block the muscle-type but not the α7 AChRs, and is closer to the long α-neurotoxins, which have comparable potency against the above-mentioned AChR types.