BmK AS, one novel neurotoxic polypeptide from Chinese scorpion Buthus martensi Karsch was labeled by (125)I with INDOGEN method. Iododerivative was used in binding experiments on mammal and insect synaptosomes. BmK AS binds specifically to a single class of non-interacting binding sites with high affinity (K(d)=1.49 nmol/L) and low capacity (B(max)=1.39 nmol/g) on rat brain synaptosomes. Likewise, BmK AS also binds specifically to a single class of non-interacting binding sites with high affinity (K(d)=0.79 nmol/L) and low capacity (B(max)=6.60 nmol/g) on cockroach nerve cord synaptosomes. It indicates that there are special target receptors for BmK AS type scorpion neurotoxins on both mammal and insect excitable cell membranes.