The invention relates to a method for inhibiting the growth of microorganisms in a latex, especially a sterically stabilised or cationic stabilised latex and to a composition for use in the method comprising a polymeric biguanide and an isothiazolinone of the Formula (1). ##STR00001## wherein: R is H, alkyl, cycloalkyl or aralkyl; and Y and Z each independently are H, halogen or C.sub.1-4-alkyl or Y and Z together with the carbon atoms to which they are attached form an optionally substituted 5 or 6 membered ring.