Berberine with and without fluconazole was tested by an agar disk diffusion assay in which clinical isolates of Candida albicans were applied onto yeast extract-peptone-dextrose agar plate. Berberine, which had no intrinsic antifungal activity at the concentration tested, exerted a powerful antifungal activity in combination of fluzonazole. Combinations of berberine and fluconazole were also tested by the checkerboard assay to determine whether they had favorable or unfavorable antifungal interactions. The MIC of fluconazole was 1.9 microg/ml when the drug was tested alone and decreased to 0.48 microg/ml in the presence of berberine concentrations of 1.9 microg/ml. However, berberine at concentrations of >1.9 microg/ml combined with a fluconazole supra-MIC (i.e., >1.9 microg/ml) eliminated the residual turbidity in the incubation wells. This endpoint fitted to the definition of MIC-0 (optically clear wells) and reflected the absence of a trailing effect, which is the result of a residual growth at fluconazole concentrations greater than the MIC.