The three imidazole antimycotics clotrimazole, miconazole, and ketoconazole all inhibit the demethylation of lanosterol to ergosterol, resulting in inhibition of growth of Saccharomyces cerevisiae; this is a fungistatic action. At higher concentrations clotrimazole and miconazole are fungicidal, whereas ketoconazole is not. The fungicidal action reflects direct membrane damage by the imidazoles. Evidence for this is that ketoconazole is markedly less active than the other imidazoles in its ability to allow methylene blue entry into cells and to disrupt liposome model membranes. The possible clinical significance of these findings is discussed.