A diagonal mapping technique for the selective isolation of cysteine-containing peptides was introduced about 10 years ago by Brown and Hartley (1, 2). Peptides are first separated by electrophoresis in one dimension on filter paper. The paper is then dried and exposed to performic acid vapors which cleave and oxidize cystine residues into two cysteic acid residues. The electrophoretic mobility of cysteic acid-containing peptides usually changes because the peptides are smaller and cysteic acid residues are negatively charged. A second electrophoresis is run at right angles to the first dimension. Peptides which do not contain cysteine have the same mobility in both dimensions and lie on a 45° diagonal. Cysteine-containing peptides “map” off the diagonal. I have developed a new version of this technique which is faster and more specific for cysteine-containing peptides. Proteins are first completely sulfitolyzed (3) and then enzymatically digested; cysteine and cystine residues are thus present as negatively charged S-sulfo cysteinyl derivatives (—S—SO3-).