The present invention relates to a method of inhibiting the growth of killing algae, bacteria, yeast and/or fungi in a recirculating water system which comprises adding to the water a polymeric biguanide and an adjuvant of the Formula (1) or a salt thereof: ##STR1##

wherein:

• R¹ is an optionally substituted C₈ to C₁₂ or C₁₈ to C₂₂ alkyl group; and
• R² and R³ each independently is H or optionally substituted C_1-4-alkyl.