The antifungal effects of amphotericin B are believed to be due to two possibly interrelated mechanisms: an increase in permeation by binding to sterols in cellular membranes and a prooxidant effect causing oxidative damage in target cells. However, the seven conjugated double bonds in amphotericin B raise the possibility that it could be highly susceptible to autoxidation, causing an antioxidant effect. In the present study, we investigated the prooxidant and antioxidant properties of amphotericin B in a model system in which oxidation of a reporter molecule, cis-parinaric acid, was induced by azo initiators of peroxyl radicals. Since interactions of amphotericin B with sterols are essential for its pharmacological and toxic actions, we also studied the effects of cholesterol on the prooxidant and antioxidant properties of amphotericin B. Amphotericin B caused a noncollisional quenching of a characteristic fluorescence of cholesteryl cis-parinarate integrated in liposomes, suggesting the formation of amphotericin B-cholesteryl cis-parinarate complex. This effect of amphotericin B was ablated by increasing concentrations of cholesterol. We found that amphotericin B inhibited oxidation of cis-parinaric acid complexed with human serum albumin [using a water-soluble azo initiator, 2,2'-azobis(2aminopropane)dihydrochloride] and in liposomes [using a lipid-soluble azo initiator, 2,2'-azobis(2,4-dimethylvaleronitrile)]. The inhibitory effect of amphotericin B on 2,2'-azobis(2,4-dimethylvaleronitrile)-induced peroxidation of cis-parinaric acid in liposomes was also diminished by cholesterol. The antioxidant effect of amphotericin B in this model system suggests that amphotericin B does not exert its pharmacological and toxicological responses through a prooxidant effect to cause damage in target cells.