Blood coagulability has been studied in 30 schistosomotic patients treated with daily 25 mg/kg of CIBA 32.644-Ba (S-(Nitro-thiazolyl-(2)-2-oxo-tetrahydro-imidazole). Prothrombin count, coagulum retraction fibrinogen dosage, blood platelet count, bleeding and coagulum time (amongst other laboratory investigations) have been carried out in all of them. The general alterations observed were slight and seem not to be capable of provoking any hemorrhagic phenomena, which might occur in patients with hepatointestinal S. mansoni under the drug. Though proihrombin time appeared slightly decreased in 6 patients during a six-month period following treatment stop, this could not cause hemorrhage, since these values did not reach the threshold level. The alterations seen in coagulum retraction may not be interpreted as a result from any changes in blood platelet physiology.