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A feed-forward control technique was used to control the concentration of ammonium during the fed-batch cultivation of Saccharomyces cerevisiae. This control system was tested by studying the growth of S. cerevisiae at different, constant ammonium concentrations. In a complex medium, as the controlled level of ammonium increased from 38 to 300 mM, the ammounium consumption increased, the protein content of the cells increased, and the biomass yield decreased. At 230 and 300 mM, the ammonium and glucose consumption appears to have supported the production of some as yet unidentified extracellular products.