The invention relates to a preservative comprising at least one isothiazolone, at least one quaternary ammonium compound, and at least one stabilizer selected from the group of sulphite salts, hydrogen sulphite salts, sulphite-releasing compounds and hydrogen-sulphite-releasing compounds, the presence of algicidal triazine being ruled out. The preservative inhibits the growth of fungi, algae and bacteria on or in alkaline coating compositions. The invention furthermore relates to a process for the preparation of the preservative, to its use in a coating composition, and to the preserved composition.