Two families, comprising 11 individuals in the toxemic form of schistosomiasis mansoni, infected in Belo Horizonte, Brazil were treated. Parasitological cure was obtained in 5 (45%) of the patients after a single oral dose of oxamniquine (Mansil®), 20 mg/kg body weight. No significant side effects were observed. To evaluate the possibility of resistance to the drug, cercariae collected from Biomphalaria glabrata infected with miracidia from eggs obtained from three of the individuals not cured were studied. Mice infected with these three strains were cured after a single oral dose of oxamniquine. It is suggested that research be continued with other therapeutic schedules and perhaps other, more potent, drugs.