A clinical trial of a new antischistosomal drug, hycanthone methanesulfonate, was undertaken in 103 patients with schistosomiasis mansoni hospitalized in St. Lucia, West Indies. The drug was administered in a single intramuscular dose of 3 mg per kg of body weight. The commonest side effect, vomiting, occurred in 48 patients (47%); 33 other patients complained of nausea, anorexia, or abdominal pain. In general, these symptoms ceased within 24 hours after treatment. Quantitative examination for Schistosoma mansoni eggs was carried out on three fecal specimens before treatment and at 6 weeks, 4 months, and 6 months after treatment. At 6 months after treatment, 26 of 94 patients (28%) were no longer excreting eggs, and 76 patients (81%) had sustained a greater than 90% reduction in egg-excretion level; for these 94 as a group, the total number of eggs excreted had been reduced by 97%. The efficacy of the drug did not appear to vary with severity of infection as measured by number of eggs excreted or by enlargement of the liver or spleen, or both. In this group of patients, two-thirds of whom were under 20 years of age, enlargement of the liver or spleen was frequently found to regress after treatment.