Nonlinear adsorption plays an important role in determining the chromatographic behavior of proteins in preparative ion-exchange chromatography. In this article, the steric mass action (SMA) isotherm is used in conjunction with a mass transport model to describe nonlinear cation-exchange chromatography. Excellent agreement is observed between simulated and experimental step gradient separations of the proteins alpha-chymotrypsinogen A, cytochrome C, and lysozyme. A systematic method of selecting the optimum step gradient program for a given separation problem is presented and employed to study optimization of step gradient chromatography under conditions of high mass loading. This article includes consideration of the effects of the adsorption properties of the feed stream, the feed stream concentration, protein solubility, and other constraints on the optimum separation conditions.