Twelve patients with ova of 5. mansoni in their stools were treated with the pentavalent antimonial drug, neostibosan (Winthrop). The patients were all males with ages ranging from 15 to 32 years. The drug was administered intravenously during a 2-week period of hospitalization. During the first 3 days, doses of 0.2, 0.6 and 0.9 respectively were usually given, and thereafter either 0.6 or 0.9 was generally injected every day until the end of hospitalization. All patients tolerated the drug well and no serious reactions were seen. A low grade fever and abdominal pain, headache, nausea or anorexia were observed in most of the patients.The effect of the drug on the number of schistosome eggs passed in stools was carefully observed. Stool examinations were made daily before and during treatment and on as many daily specimens as possible for each follow-up examination. The technic used for the quantitative determination of eggs in feces has been described previously.1The effect of treatment on the schistosome...