Abstract: It has been reported that pumpkin seeds (Cucurbita moschata Duch.) possess protective activity against schistosomal infection. The present paper deals with investigations on its active principle. In collaboration with the Institute of Materia Medica, Academia Sinica, about 80 extracts from pumpkin seeds were screened in experimentally infected mice for their activity.It was found that cucurbitine, a new amino acid, was the most active. With this active principle, a series of experiments was carried out on white mice. When mice, each infected with 58—62 cercariae of Schistosoma japonicum, were administered daily per os with cucurbitine 350—400 mg/kg for 28 days since the day of infection, schistosomulae were much retarded in their development, with a worm reduction rate of 43.7—68.5%. When the same dosage was administered to infected mice 2—3 weeks after cxposure, the prophylactic activity was not evident. No difference in efficacy was observed among groups of mice infected with 20, 40, 60 and 100 cercariac. When mice, each infected with 40 cercariae, were treated 4 weeks later with cucurbitine (250 mg/kg×7 days+500mg/kg×21 days), adult worms were decreased in size and their reproductive organs were degenerated. But no dead worm was found. In infected mice treated with cucurbitine 4 weeks after exposure in the dosage of 350 mg/kg for 28 days, histochemical observations on RNA, DNA, alkaline phosphatase and acid phosphatase of the worms showed no remarkable change. During the treatment, pathological lesions caused by cucurbitine on the host tissues were rather insignificant.