The invention relates to a biocide composition for use as an additive in substances which are susceptible to attack by harmful micro-organisms. The inventive biocide composition has at least two active biocide agents of which one is 2-methylisothiazolin-3-on, and is characterised in that it contains 1,2-benzisothiazolin-3-on, compositions containing 5-chloro-2-methylisothiazolin-3-on being excluded. The inventive composition has a synergistic biocide effect compared to its individual constituents.
