The capacity of four culture media to obfuscate the antifungal activity of miconazole and amphotericin B methyl ester was evaluated qualitatively by examination of five isolates each of Candida albicans, Candida tropicalis, Candida parapsilosis, Torulopsis glabrata, and Cryptococcus neoformans, and quantitatively by determination of the absolute minimal inhibitory concentrations for a strain of C. albicans. Miconazole, like the predecessor imidazole (clotrimazole), was antagonized by two complex, undefined media (Sabouraud's glucose and brain-heart infusion agars) but not by either of two synthetic formulations (synthetic amino acid medium, fungal and modified yeast-nitrogen base). The antifungal activity of amphotericin B methyl ester, like that of the parent compound amphotericin B, was not materially affected by the culture medium used for testing. When added separately to synthetic amino acid medium (fungal), neither purines nor pyrimidines antagonized miconazole. Ether extraction of Sabouraud's glucose agar failed to diminish the antagonism of this medium for miconazole.