A synergistic antimicrobial composition containing a glyphosate compound and at least one of 4,5-dichloro-2-n-octyl-4-isothiazolin-3-one or 2-n-octly-4-isothiazolin-3-one or N-Butyl-1,2-benzisothiazolin-3-one is provided. Also provided is a method of inhibiting the growth of or controlling the growth of microorganisms in a building material by adding such a synergistic antimicrobial composition. Also provided is a coating composition containing such a synergistic antimicrobial composition, and a dry film made from such a coating composition.
