Clinical trials with hycanthone, an active metabolite of lucanthone, were performed in 203 patients with active schistosomiasis mansoni. The drug was administered in the form of enterocoated tablets and by intramuscular injection (sulfamate and methanesulfonate salts). Anorexia, nausea, vomiting, abdominal pain, headache, muscle pain, and loss of body weight were the commonest side-effects. In all but six patients, treatment with enterocoated tablets could be completed. With the parenteral formulations no interruption of treatment was necessary. Alterations of plasma-level transminases and of electro-cardiographic tracings were observed in a few patients. The most promising schedule of treatment seems to be a single intramuscular dose of the methanesulfonate salt of hycanthone (2 to 3 mg per kg) since the cure rate was higher than 90% without appreciable side-effects. Further clinical trials and pilot projects for mass treatment should be encouraged.