Abstract: Both tartar emetic and sodium antimony gluconate contain minute amounts of arsenic. The permissible limits of the arsenic content of the 2 drugs differ in different pharmacopoeias. In order to find the safety limit of arsenic, a systematic study on the effects of minute amounts of arsenic on the toxicity of antimonials was undertaken in mice. The results indicate that if the arsenic contents were below 200 ppm, no significant influence on the toxicity of the 2 antimonials was found. The toxicity of the 2 antimonials began to increase only when the arsenic content was increased to 2000—20000 ppm. By analyzing the toxicity of the combined use of tartar emetic (PAT_sb^Ⅲ) with arsenic trioxide (AS^Ⅲ), and sodium antimony gluconate (SAG_sb^v) with disodium hydrogen arsenate (AS^v) in mice, according to the method of Loewe, we found that there was an additional effect in their toxicities.