One hundred patients with active schistosomiasis mansoni were treated with niridazole (25 mg/kg/day po for 7 days). The most serious side effects were those connected with the neuropsychic area (convulsion, hallucination, etc.). Therapeutic activity, based on repeated stool examinations, was about 90% in adults and 60% in children. Because of the side effects, as well as the long-term schedule of treatment, niridazole is not recommended for use under field conditions. Seven hundred and forty-five patients were treated with hycanthone (2.5 mg/kg im) after clinical examination. The side effects observed were mild and of short duration (24-48 hr). The therapeutic activity was very high (about 95% cure). So far we have treated more than 9,000 infected patients without severe complications, including hepatic failure and death, which have been reported by some authors. A hycanthone-resistant strain has been isolated from two patients unsuccessfully treated twice with hycanthone and once with niridazole. Considering that an important aspect of the control of schistosomiasis is mass treatment, further studies regarding mutagenicity, teratogenicity, resistance, and toxicological effects of hycanthone are still required before this drug can be recommended for wide use.