Abstract: The active principle of Hemerocallis thunbergii Baker was isolated by extraction from the pulverized drug with chloroform in Soxhlet apparatus. By chromatography on aluminium oxide, a yellow powder (sample Ⅰ) was obtained, which became brownish and sintered above 243℃, and finally melted at 266—269℃ under decomposition. Its LD₅₀ to mice was 0.95 mg/20 g body weight. Recrystallization from dimethyl formamide yielded orange red crystals (sample Ⅱ), which also became brownish, sintered above 240℃ and melted at 268—269℃ under decomposition. Its effect and toxicity were greatly dropped; the LD₅₀ could not be determined below 60 mg/20g. But the effect and the toxicity reappeared when the dose was greatly increased. Dissolve the orange red crystals in sodium hydroxide solution and then acidify by diluted hydrochloric acid, a yellow powder (sample Ⅲ) was again obtained, which also became brownish, sintered above 240℃, and melted at 268—269℃ under decomposition. Its LD₅₀ was 0.34 mg/20 g. The results of experimental therapy of schistosomiasis japonica in mice indicated that the effect of the drug was parallel to its toxicity. The samples Ⅰ,Ⅱ and Ⅲ showed no depression on the determination of the mixed melting point. Their infrared spectrum and that of their acetyl derivatives were all identical. Paper chromatographic tests with three different solvent systems showed in each case only one reddish-violet spot on spraying with ferric chloride reagent. These results indicated that the samples, Ⅰ, Ⅱ and Ⅲ may be the one and same compound whose empirical formula is C₁₆H₁₄O₄. The name "hemerocallin" was suggested for the active principle obtained.