The cholinesterase activity of horse serum is strongly inhibited by phenothiazone and by some phenothiazine alkyl-sulphonium salts. The magnitude of this inhibition is about equal to that produced by methylene blue and by acriflavine. Alkaline phosphatase and serum lipase are not affected.The possible relation of this property of phenothiazine derivatives to certain nervous symptoms observed in treated pigs, and to insecticidal and anthelmintic action, is discussed.