In a new procedure for the treatment of a cooling water system or of a water system used in paper mfr., glutaraldehyde and an isothiazolone are added directly or indirectly. New compsns. suitable for use as additives to an aq. system contain an isothiazolone and glutaraldehyde in a ratio of 1:1 to 1:10 (pref. 1:3 to 1:5, esp. ca 1:4) w/w. - Biocides to prevent the growth of slime-forming bacteria in circulating water systems. The two components act synergistically, so that smaller amts. of the relatively expensive isothiazolones can be used. (0/0) DEAB- DE3313689 C In a new procedure for the treatment of a cooling water system or of a water system used in paper mfr., glutaraldehyde and an isothiazolone are added directly or indirectly. New compsns. suitable for use as additives to an aq. system contain an isothiazolone and glutaraldehyde in a ratio of 1:1 to 1:10 (pref. 1:3 to 1:5, esp. ca 1:4) w/w. - Biocides to prevent the growth of slime-forming bacteria in circulating water systems. The two components ac