Cobra venom may be inactivated by NaHSO3 at pH 4.5 and by H2SO3 at pH 2. For comparison, the action of H2SO3 on other compounds containing cystine groups, such as l-cystine, S-S-glutathione, egg albumin and insulin, was studied. Cysteine-sulfo acid was obtained by dissolving 3.0 g. of 1-cystine in 1 L.H2O and by introducing SO2. After 72 hrs., the mixture was conc. acidified with H2SO4, and cysteinesulfo acid (C3H7O5NS2) obtained, m.p. 184o-185o. Upon hydrolysis with HC1 and acetic acid, the cysteinesulfo acid gave H2SO4 and cysteine. When treated with H2SO3 in a similar manner, cobra venom neurotoxin formed a product insoluble in H2O and one soluble in H2O. When hydrolyzed, these products gave H2SO3 and H2SO4. Under similar conditions, egg albumin, insulin and a protein obtained from Antiaris toxicara gave only H2SO4. The poisonous property of snake venom is possibly due to the presence of a prosthetic group in the molecule.