β₂-Bungarotoxin (β₂-toxin) was isolated from the venom of Bungarus multicinctus by means of CM-Sephadex C-25 column chromatography, Sephadex G-75 gel filtration and CM-Sephadex C-25 column rechromatography. β₂-Toxin consisted of two dissimilar polypeptides, A (120 amino acid residues) and B (60 amino acid residues) chains, crosslinked by an interchain disulfide bond. The neurotoxicity (LD₅₀ and phospholipase activity of β₂-toxin were 0.029 μg/g of mouse and 48.9 unitsμ/mg of toxin, respectively, and both the activities were slightly weaker than those (0.019 μg/g and 60.9 units/mg) of β₁-bungarotoxin (β₁-toxin).β₂-Toxin was reduced and carboxymethylated and then its RCM-A and -B chains were separated. Each RCM-chain was maleylated and then digested with TPCK-trypsin. The tryptic peptides were sequenced by manual Edman degradation or the dansyl-Edman method, and the total alignment of the tryptic peptides from each RCM-chain was deduced based on the amino acid sequences of the A and B chains of β₂-toxin. The amino acid sequence of the B chain of β₂-toxin differed from that of the B chain of β₁-toxin by 22 amino acid substitutions, while those of their A chains were identical. We concluded that the variation in the amino acid sequence of the B chains did not significantly affect the neurotoxicity of the β-toxins.The amino acid sequences of the B chains of the two β-toxins were homologous to those of proteinase inhibitors from snake venoms and mammalian pancreas, but no inhibitory activity of the two β-toxins on proteinases was observed.