Abstract: It was found that F-30066, a new furacin derivative, possessed a direct effect on schistosomes in vitro. Trials were made with sheep serum-Tyrode solution (1:2) containing 0.5% glucose. When parasites were kept in the medium containing 4— 10 μg/ml of F-30066, the motility of the worms decreased gradually and the shrinkage of the worm body followed. After 24 hours, only slow body movement in the female and slow sucker movement in the male were observed. At the same time the reproductive organs of the worms degenerated and the number of ova in the uteri was markedly reduced. Within 48 hours, both male and female worms were killed after a long period of paralysis. In order to explore the effect of F-30066 on the survival of worms in vivo, mice infected with schistosomes were treated with F-30066 or tartar emetic in the dosage of 1/2 LD₅₀ per os for 5 days. Each day worms were removed from 2—3 mice and maintained in the medium. In case of tartar emetic, the motility of the worms recovered gradually and the worms survived as long as those from the control group. In case of F-30066, the worms became sluggish and could not recover completely. After the 4th and the 5th dose, the worms survived in the medium for only 4.2—7.0 days, while the survival time of the worms from control group was 12.4 days. The difference is statistically significant. When worms were placed in serum taken from rabbits 1/2 to 8 hours after a single oral dosage of 1.3gm/kg of F-30066, the survival time of the parasites was reduced from 12.2—12.8 days to 3.2—9.1 days. It was found furthermore that the antischistosomal activity of F-30066 in vitro could be antagonized by cysteine, but not by cystine, lipoic acid, sodium dimercaptosuccinate, or vitamine C.