Affinity sorption is an effective and widely used method for purification of proteins and protein complexes. Usually no internal standard is included in the final analysis despite a significant potential for intra-experimental error in this multistep procedure. In this study we suggest a robust internal standard for protein purification by affinity sorption. The internal standard consisted of biotinylated bovine serum albumin (BSA), which is included in the samples prior to the purification procedure. The amount of co-purified BSA has shown linearity within a wide range of concentrations and was not influenced by the variations in the amount of the target protein.