The invention relates to an additive for hydraulically setting systems based on modified polycarboxylates and water-soluble ethers of high polymeric polysaccharides, optionally containing further standard additives, characterized by a content of a) a water-soluble ether of cellulose or a cellulose-like compound with a viscosity of at least approximately 1,000 mPas, particularly at least approximately 2,000 mPas, measured as a 2% aqueous solution with a Brookfield viscosimeter at 20.degree. C. and 20 rpm and b) a polycarboxylate, whose main chain is linked by means of ester, ether, imide and/or amide groups with polyethylene oxide-containing side chains. The invention also relates to hydraulic mixtures having a content of the above additive and the use thereof. Using the additive according to the invention in hydraulically setting systems comparable and in part much better characteristics are obtained compared with casein-containing formulations, such as a rheological flow behaviour comparable to casein, but with improved water retention.