Ultrastructural changes in Candida albicans induced by increasing concentrations of miconazole in vitro are described. Fungistatic concentrations (10(-8) to 10(-7) M) induced minimal morphologic changes at the cell periphery. At 10(-6) M cell volume increased and peroxisomes became numerous in the cell interior. The minimal fungicidal dose of 10(-5) M caused severe damage to most of the cell population and a total fungicidal dose of 10(-4) M caused total internal cellular necrosis even when cell walls remained intact. It is suggested that miconazole inhibits the peroxidative enzymes cytochrome c-peroxidase and catalase. Cell necrosis then results from peroxide accumulation.