1 Phase 1 and 2 studies were carried out in thirteen volunteers and thirty patients with hookworm anaemia with a new anthelmintic, 4-isothiocyanato-4'-nitrodiphenylamine (C.9333-Go/CGP 4540). Single and multiple doses were administered by mouth under careful supervision by the staff of the clinical pharmacology unit. 2 4-Isothiocyanato-4'-nitrodiphenylamine (C.9333-Go/CGP 4540) was well tolerated in volunteers up to a single dose of 1750 mg and in patients up to 3000 mg in divided doses. Side effects were rare, transient and mild. One patient had moderate giddiness for 24 h after the compound. The laboratory investigations did not show any adverse effects on organ functions. Transient non-specific changes in the T-wave were occasionally observed in the e.c.g. 3 Multiple doses (125 mg, 250 mg or 1000 mg X 3) of the compound were highly effective in eradicating hookworm infection, as judged by almost 100% reduction in faecal egg counts and significant recovery of parasites from 24 h stool samples. The compound also showed activity against A. lumbricoides, T. trichiura and E. vermicularis. The compound appears to be promising and more studies, in a field situation, are desirable.