A simple modification of the conventional one-dimensional sodium dodecyl sulfate--polyacrylamide gel electrophoresis technique has been used to visualize inter- and intramolecular disulfide bonding in proteins. The gradient of reducing agent established between adjacent slab gel tracks by electrophoresing identical protein samples next to one another, one containing and the other not containing 2-mercaptoethanol, has been used to visualize the change in mobility of disulfide bond-containing proteins throughout the transition from a reducing to a nonreducing environment. As illustrated by an analysis of immunoglobulin heavy and light chains, the method particularly facilitates the positive identification of proteins containing intrachain disulfide bonds.