We compared the in vitro antifungal action of amphotericin B (AmB) used alone or in combination with a second polyene antibiotic or with miconazole or ketoconazole. When AmB was used in combination with either filipin or Etruscomycin (Farmitalia, Milan, Italy), antagonism or potentiation of the antifungal effect against Candida albicans resulted. Addition of AmB to Etruscomycin- or filipin-treated cultures resulted in antagonism. In contrast, potentiation occurred when Etruscomycin or filipin was added to cultures treated with AmB. The outcome of incubating C. albicans with combinations of AmB and either miconazole or ketoconazole depended on the duration of exposure of the cells to the drugs. Short-term incubations resulted in antagonism, whereas potentiation of antifungal effects occurred after prolonged exposure of cells to the antibiotics. In addition, supplementation of cultures with serum protein-potentiated AmB induced k+ leakage at low protein concentrations and inhibited K+ leakage at high protein concentrations.