The positions of the disulfides bonds in Cerebratulus lacteus toxin A-III were investigated by hydrolysis of the unreduced protein with trypsin. The resulting peptides were purified by gel filtration, paper electrophoresis, and paper chromatography. Determination of the amino acid compositions of the purified peptides demonstrated the existence of disulfide bonds linking half-cystine residues 17 and 38, 23 and 34, and 48 and 61.