The amodiaquine metabolite 2-hydroxydesethylamodiaquine (designated metabolite II), one of the two major human metabolites of this antimalarial prodrug, is characterized by chromatographic and spectroscopic methods. This metabolite has been isolated in milligram quantities from the urine of an amodiaquine-dosed individual by extraction and preparative high-performance liquid chromatography (HPLC) and standardized using nuclear magnetic resonance spectroscopy with internal standardization. Aliquots of this standard provided accurately known amounts of the compound for spectroscopic characterization, for use as an HPLC standard and for assessment of in vitro activity against malaria parasites. Knowledge of the structure of the two major metabolites of amodiaquine (the other is desethylamodiaquine) permits speculation as to the presence of three additional human metabolites, chromatographic confirmation for one of which is demonstrated. The in vitro activity of metabolite II is shown to be 1% that of amodiaquine for two chloroquine-sensitive Plasmodium falciparum strains. Should this relationship hold generally, desethylamodiaquine is the only chemical species resulting from oral dosing with amodiaquine which contributes significantly to antimalarial activity in the blood.