The present invention relates to certain processes and compositions useful for inhibiting the growth of slime in water and, in particular, water used for industrial purposes; for example, in the manufacture of pulp paper, in the manufacture of paper, in cooling water systems and in effluent water treatment. The novel processes and compositions of the present invention are processes or mixtures which show unexpected synergistic activity against microorganisms, including bacteria, fungi, and algae, which produce slime in aqueous systems or bodies which are objectionable from either an operational or aesthetic point of view. Specifically, the invention is directed to the use of compositions comprising a combination of a mixture of 75% of 5chloro-2-methyl-4-isothiazolin-3-one calcium chloride and 25% 2methyl-4-isothiazolin-3-one calcium chloride and octyl-phenoxy polyethoxy ethanol.