Abstract: By single intraperitoneal injection to wwhite mice, the LD₅₀ of tartar emetic, 1,2-dihydroxy benzene 3,5-disulphonic acid and its sodium salt were found to be 0.8, 101.5, and 145.1 mg/20g respectively. When 1,2-dihydroxy benzene 3,5-disulphonic acid or its sodium salt was injected intraperitoneally into mice immediately after the administration of tartar emetic, the toxicity of the latter is markedly reduced. The LD₅₀ of tartar emetic could be raised from 0.8 to 10.6 mg/20 g. The antidotal effect of the two drugs against tartar emetic was evident.The compound 1,2-dihydroxy benzene 3,5-disulphonic acid can protect guinca-pig against the local irritation induced by tartar emetic. The therapeutic activity of tartar emetic against Schistosomiasis japonica in mice was diminished by Na-l,2-dihydroxy benzene 3,5-disulphonate. At the dosage of 8.0 mg/20g Na-1,2-dihydroxy benzene 3,5-disulphonate would render the tartar emetic at the dosage of 0.4mg/20g completcly ineffective.