Paper, board and cardboard are produced by draining paper stock containing interfering substances in the presence of fixing agents by a process in which the fixing agents used are reaction products which are obtainable by reacting amino- and/or ammonium-containing polymers selected from the group consisting of the polymers containing vinylamine units polyalkylene polyamines polyamidoamines ethyleneimine-grafted polyamidoamines which may be crosslinked, polydiallyldimethylammonium chlorides polymers containing dialkylaminoalkylacrylamide units or dialkylaminoalkylmethacrylamide units and polyallylamines and dicyandiamide and formaldehyde condensates with reactive sizes for paper in a weight ratio of polymer to reactive size of from 15,000:1 to 1:1, and the reaction products thus obtainable are used as fixing agents for water-soluble and for water-insoluble interfering substances in the production of paper, board and cardboard from paper stocks containing interfering substances.