The present invention relates to a method of inhibiting the growth of microorganisms in a metal working fluid comprising adding to the metal working fluid a benzisothiazolin-3-one of the Formula (1): wherein R is C4-5-alkyl; R<;1 >;is hydroxy, halogen (especially chlorine), C1-4-alkyl or C1-4-alkoxy; and n is from 0 to 4.