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The S-sulfonated derivatives of the A and B chains (A-SSOr and B-SSOr) of bovine and porcine insulins were prepared by sulfitolysis of the respective natural hormones. The separation and purification of the so-produced A-SSO.r and B-SSOr were accomplished by three different methods: (a) continuous-flow electrophoresis, (b) chromatography on carboxymethylcellulose columns, and (c) chromatography on aminoethylcellulose columns. Identical material was obtained by all three methods as judged by amino acid analyses of acid and enzymatic hydrolysates, paper and thin layer electrophoresis, and infrared spectroscopic analysis. The over-all yield of the purified chains, based on the amount of insulin sulfitolyzed, was 60-70 % for the T A he molecule of insulin is composed of two polypeptide chains, A and B, linked together by two disulfide bridges. In addition, there is an intrachain disulfide bond in the A chain (Sanger and Tuppy, 1951a,b; Sanger and Thompson 1953a,b). Upon treatment of insulin with sodium sulfite in the presence of a mild oxidizing agent (sulfitolysis), its disulfide bonds are broken and the A and B chains are converted to the corresponding S-sulfonated derivatives (Swan, 1957; Bailey and Cole, 1959). The separation and purification of the S-sulfonated derivatives of the A and B (A-SSO.r,

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