The amino groups of Laticauda semifasciata III (LsIII) purified from the venom of a sea snake, Laticauda semifasciata, were acetylated with acetic anhydride. Three monoacetyl derivatives of LsIII, namely, [1-N2-acetyl-arginine]-LsIII, [23-N6-acetyl-lysine]-LsIII and [35-N6-acetyl-lysine]-LsIII, were obtained. These monoacetyl derivatives of LsIII showed the same CD spectra as that of native LsIII. [1-N2-Acetyl-arginine]-LsIII was half as active as the original toxin suggesting the importance of the net positive charge of the toxin molecule for the toxicity. [23-N6-Acetyl-lysine]-LsIII and [35-N6-acetyl-lysine]-LsIII showed no toxicity. Lysine-23 is one of the residues common to all neurotoxins, whereas lysine-35 is one of the residues found specifically among long-chain neurotoxins, although there are some exceptional toxins without lysine-23 or lysine-35.