The chromatographic fractionation of proteins by size-exclusion chromatography in a simulated moving bed (SMB) is studied. During experimental fractionation of a mixture of bovine serum albumin (BSA) and myoglobin on Sepharose Big Beads, mass-transfer effects are shown to limit the performance of the SMB. The internal profiles, as well as the extract and raffinate compositions, are described well by a steady-state equivalent true moving-bed (TMB) model that incorporates mass-transfer effects. The selection of the particle size in SMB is a trade-off between productivity and mass transfer. Based on the equivalent TMB model, the optimum particle size and configuration of the SMB can be selected, at which preset performance criteria (purity, recovery) are met at specified flow-rate ratios, total column length, and pressure drop. For the current feed and apparatus, an optimal particle size of approximately 145 μm is calculated for achievement of purities and overall recoveries of 95%.