Abstract: Dipterex is an organophosphorus insecticide, characterized by its anticholinesterase property. The present study is concerned with its toxicity and experimental therapy for schistosomiasis japonica, as well as the antidotal effects of atropine and PAM. The intragastric and subcutaneous LD₅₀ of dipterex in mice were found to be 0.8 and 0.6 g/kg, respectively. In mice, concomitant injections of atropine and PAM 1/2 hour prior to dipterex yielded a better detoxicating effect than either antidote given alone. Rabbits receiving dipterex 30 or 60 mg/kg/day by stomach tube for 2 weeks showed a 70%-inhibition of plasma cholinesterase. Monkeys were daily fed dipterex, the dosage of which being started from 4 mg/kg and increased by 4 mg/kg successively each day. On the 6th day the monkeys refused food and the cholinesterase activity was depressed markedly. They collapsed on the 7th day, but recovered 5 days after discontinuance of dipterex. Mice infected with Schistosoma japonicum were fed dipterex 0.2 g/kg/day for 2 weeks. The average number of worms remained in each mouse of the treated group was found to be 13± 5, and that of the control group 24±6. The difference was highly significant. Infected rabbits receiving daily 30 mg/kg intragastrically or 40 mg/kg hypodermically for 2 weeks also showed a reduction of the number of helminths. Five infected dogs were daily fed dipterex for 2 weeks. All the fecal examinations for ova became no longer positive. After a holding period of 2 weeks, the average number of worms remained in each dog was reduced to 8±9, whereas that of the 6 control dogs was 47±29. The difference was significant. During the course of treatment in dogs, there appeared no conspicuous alterations in serum BSP retention and blood picture. The plasma cholinesterase activity fell to 25% of the original level, and restored to 75% subsequent to a 2-week holding period. The combined therapy with dipterex and tartar emetic resulted in a better effect than either drug alone. Atropine and PAM did not attenuate the therapeutic efficacy of dipterex. In conclusion, definite therapeutic action of dipterex for schistosomiasis japonica was revealed. It is cheap and can be taken by mouth. Despite certain toxic reactions, dipterex has surely opened up a new terrain for searching non-antimonial remedies.