Mammalian catalase and cytochrome oxidase are strongly inhibited by the hydroxy derivatives of phenothiazine and by p-hydroxylaminobenzenesulphonamide. Phenothiazine, sulphanilamide, and sulphapyridine have little or no effect. Cytochrome-c is irreversibly reduced by the hydroxy-sulphanilamide, as indicated by spectroscopic observation. The inhibitory activity apparently depends on the presence of the hydroxyl group. The relation of these findings to the vermicidal and bactericidal action of the compounds is discussed.Phenothiazine and thionol have no effect on Ascaris lumbricoides in vitro.