The invention relates to short 3- to 10-mer bioactive oligopeptides composed of amino acid residues selected from the group consisting of glycine, alanine, β-alanine, serine, histidine, glutamic acid, aspartic acid, lysine and arginine. They are able to improve at least one cell culture parameter such as growth rate, viable cell number, viability on day 6 or 7, or final product yield, when added to the basal nutrient medium. The invention further relates to mixtures of such oligopeptides and to a method of improving cell culture parameters by incubating animal or plant cells in the presence of said oligopeptides.