This invention relates to synergistic combinations of selected isothiazolin-3-one microbicides in combination with a second microbicide or formulation ingredient or raw material. The combinations have greater efficacy than would be expected from combinations of the individual components. The combinations include mixtures of (a) Methyl-4-isothiazolin-3-one or (b) 1,2-Benzisothiazolin-3-one with one or more of a variety of other compounds.
