The purpose of this investigation was to determine the cytotoxicity of amphotericin B (AMB; trade name Fungizone [FZ]) following the administration of FZ and a heat-treated form of FZ (HFZ) to LLC-PK1 pig kidney cells and Cryptococcus neoformans var. gattii cells. HFZ was significantly less toxic to kidney cells than FZ at all concentrations tested. For both FZ and HFZ, the concentration range which resulted in a 50% reduction of the growth of fungal cells was 0.125 to 1 mg/ml. These findings suggest that heat treatment decreases AMB's renal cytotoxicity without modifying its antifungal activity.