Abstract: A method is provided of treating an air-contacted surface of an air conditioning system comprising the steps of:(A) introducing to the air passage and air-borne biocide in a biocidally effective amount; and(B) subsequently introducing into the air passage an air-borne biostat adapted to coat the surface with an air-driable, substantially water insoluble biostat coating, the biostat being introduced in an amount sufficient to coat the surface with a biostatically effective coating. According to another aspect of the invention, a kit is provided of interrelated parts for carrying out such treatment method.