The anthelmintic activity of Amoscanate (C 9333-GO/CGP 4540) has been studied in experimental infections with the human hookworm, Necator americanus, in hamsters; Nematospiroides dubius, Hymenolepsis nana and natural infections with Syphacia obvelata in mice; Ancylostoma caninum, A. ceylanicum in mongrel dogs; Oesophagostomum apiostumum and Strongyloides fuelleborni in rhesus monkeys. Single oral doses of 30-60 mg/kg eliminated 94 to 99% of the total Necator parasites in 37-day-old non-patent infection, while single oral doses of 25 mg/kg expelled the entire worm burden in patient infection in hamsters. When incorporated in feed at the 0.01% level and administered ad lib. to hamsters at 20, 27 and 34 days postexposure for 5 days, the worm burdens were reduced 88 to 94%. N. dubius was completely eradicated by a single oral dose of 200 mg/kg while S. obvelata and H. nana were expelled at doses of 12.6 mg/kg and 50 mg/kg, respectively. A dose of 25 mg/kg in dogs produced 100% fecal egg reduction in hookworm and 99% in ascarid infections. The drug at 3 x 2.5 mg/kg administered at 4-hour intervals produced similar effects in mixed A. caninum and A. ceylanicum infections. Against natural infections of S. fulleborni and Oe. apiostumum in rhesus monkeys the drug showed 100% efficacy at a dosage of 60 mg/kg administered thrice at 12-hour intervals.