Abstract: Hemerocallis thunbergii Baker is a traditional drug, which has been proved to be schistosomicidal in experimental infection but very toxic to the host. In order to explore its toxicity and methods of detoxication, a series of experiments was carried out on albino mice. It was determined that the acute LD₅₀ of its root and cortex of root were 82 and 49 mg/20 g body weight respectively. No toxic reactions were observed when either rhizome remnant of scalar vascular bundles or vascular cylinder of root was administered in dosages of 140, 160 and 220 mg/20g respectively. Within the range of 22—35℃, the variation in room temperature bears no relationship to the toxicity of Hemerocallis thunbergii. The accumulative action of Hemerocallis thunbergii was very obvious and tolerance of the infected mice to the drug was even lower than that of non-infected animals. According to traditional practice, the toxicity of the drug would be decreased when placed into rice washing fluid. Experiments had been made by immersing the drug into such fluid at 20℃ or 60—70℃, but failed to achieve the detoxication. On the other hand, Coptis chinensis and Phellodendon amurenese are effective detoxicants, especially when the former was administered simultaneously with Hemerocallis thunbergii.