A new neurotoxic component named BmK abT was purified from the venom of Chinese scorpion Buthus martensi Karsch. The molecular weight of BmK abT was determined to be 7212 Da on a mass spectrum. The minimum lethal dose of BmK abT was tested to be about 1.5 microg per mouse by intracerebroventricular injection, and the dose induced significant paralysis effect on cockroach was about 5 microg by i.p. injection. The partial amino acid sequence indicated that it was a distinctive polypeptide in the scorpion neurotoxin family. Thereafter, the whole amino acid sequence of mature BmK abT was deduced from cDNA sequence by 5'- and 3'-rapid amplification of cDNA ends. Finally, it was defined to be composed of 63 residues with amidation at the C-terminal residue. By sequence comparison, BmK abT was found to be most similar to Ts VII, a beta-toxin from the New World scorpion. The patch-clamp recording on DRG neurons, unexpectedly, showed this toxin could prolong the action potential and increase the amplitude of the peak Na+ currents, which are the typical characters of alpha-toxin. These results suggested that BmK abT was a new toxic component found in the Old World scorpion species structurally similar to beta-toxins, but functionally similar to alpha-toxins.