Abstract: A synergistic combination of a water-soluble, antibacterial quaternary ammonium compound and a water-soluble oxidizing agent is found to improve effectiveness in controlling bacterial growth in swimming pool water, particularly hard water.

Abstract: A concentrated bacterial culture, capable of being cooled to temperatures as low as about -40.degree. C. for storage without rapid freezing and with minimum damage to the bacterial cells, is prepared by diluting a conventionally prepared concentrated cell paste with a liquid anti-freeze agent containing one or more water freezing point depressants which are water-soluble, are non-injurious to the bacteria, and do not form crystals when cooled to a predetermined temperature within the range of about 5 to about -40.degree. C. The amount of the freezing point depressant(s) is sufficient to prevent formation of ice crystals from the water present in the diluted product when cooled to the predetermined temperature. The culture, which does not become hard or crystalline upon being cooled to temperatures as low as -40.degree. C., can be warmed to a temperature convenient for sampling, assaying and blending and then re-cooled to a cold storage temperature without an appreciable reduction in viability.