A new anthelmintic, amidantel(Bay d 8815), an acetylated p-amino-phenyl-acetamidine was tried in 140 patients with Ancylostoma duodenale and other helminth infections. In the first trial, each 16 cases in 64 patients with A. duodenale were treated with 3.0, 6.0 and 9.0 or 10.0 mg/kg body weight of amidantel including placebo control. Another 76 patients infected with hookworms and other helminths were treated with 5.0, 6.0 and 8.0 mg/kg body weight of amidantel in the second trial. In order to assess the efficacy and safety of the drug, follow-up examination by repeated and replicated examinations over three consecutive days were performed at 14 to 16 days and 28 to 30 days after treatment, And complete laboratory studies including ECG were carried out before and one day after the medication. In the results, it was confirmed that amidantel is very effective against A. duodenale as well as Ascaris lumbricoides. With regard to dosage, a single dose of 6.0 mg/kg body weight of amidantel was found to be the most effective and well tolerated than the other dosages employed. In a single dose of 6.0 mg/kg body weight the cure rates were 93.8 and 96.6 per cent for A. duodenale infection and 90.9 and 93.1 per cent for ascariasis in the first and second trials respectivley. Relatively significant activity was also observed against Necator americanus at the dosages employed, however it was not superior to other drugs currently use. No significant activity was noted against Trichuris trichiura. Side effects including headache, nausea, dizziness and abdominal discomfort were usually mild and transient. No significant changes attributable to therapy were observed in hematology, blood biochemistry and urinalysis as well as ECG.