Staphylococcus aureus cultivated in liquid media containing untreated cornsteep liquor (CSL) and alkali-treated CSL produced similar biomass yields (6.5-6.9 g/L). However, contents of protein A in the biomass was 0.5% and 1.56% for untreated CSL and treated CSL, respectively. Addition of treated CSL at 20 g/L achieved optimal enhancement of protein A production (0.11 g/L). Probable factors associated in treated CSL for the enhanced protein A production are discussed.