The influence of chemical modification on neurotoxin RTX-III toxicity in mice has been studied. The toxicity was not affected by modification of Trp30 residue with 2-hydroxy-5-nitrobenzyl bromide but was diminished by a factor of 100 after reduction of the toxin's two disulfide bonds with 2-mercaptoethanol followed by derivatization with iodoacetamide. Blocking carboxyl groups with [3H]glycine methyl ester in the presence of 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide led to only a two-fold drop in toxicity in the case of monocarboxylate-modified derivatives and a six-fold decrease for dimodified derivatives. A conception of multipoint attachment of the toxin to sodium channel is discussed.