Antimicrobial compositions based on the synergistic combination of 2-methyl-3-isothiazolone and selected commercial microbicides where the compositions are substantially free of halogenated 3-isothiazolone, are disclosed. Particularly preferred are combinations of 2-methyl-3-isothiazolone together with benzoic acid, citric acid, sorbic acid, 1,2-dibromo-2,4-dicyanobutane, 1,3-dimethylol-5,5-dimethylhydantoin, phenoxyethanol, benzyl alcohol, zinc pyrithione or climbazole, that provide enhanced microbicidal efficacy at a combined active ingredient level lower than that of the combined individual 3-isothiazolone and commercial microbicide effective use levels.