Legionella pneumophila suspended in tap water was exposed to biocides recommended for inhibiting biological growth in cooling towers and evaporative condensers of air-conditioning systems. Chlorine, 2,2-dibromo-3-nitrilopropionamide, and a compound containing didecyldimethylammonium chloride and isopropanol were effective in destroying concentratiois of 10(5) to 10(6) viable cells per ml. Formulations consisting of 5-chloro-2-methyl-4-isothiazolin-3-one and 2-methyl-4-isothiazolin-3-one, disodium ethylene bis(thiocarbamate) and sodium dimethyl dithiocarbamate, and a phenolic with pentachlorophenate and sodium salts of other chlorophenols were less effective.