The author studied the hemolytic action of phenothiazine derivatives such as promazine, prochlorper azine, perphenazine and thiethylperazine on rabbit erythrocytes, and obtained the following results: 1. Promazine, prochlorperazine, perphenazine and thiethylperazine caused hemolysis in vitro in the following order: Thiethylperazine>perphenazine>prochlorperazine>promazine. 2. Cholesterol inhibited the hemolytic action of prochlorperazine, perphenazine and thiethylperazine, but had no effect on promazine hemolysis. 3. Dextrose, albumin and blood plasma protected erythrocytes against promazine, prochlorperazine, perphenazine and thiethylperazine. 4. The intravenous injection of promazine, prochlorperazine, perphenazine and thiethylperazine caused hemolysis in the same order as they did in vitro.