Ketoconazole, an antifungal drug, completely inhibited the growth of Candida albicans 7N at concentrations of . 50 ,ug/ml (94 ,uM). However, ketoconazole incompletely inhibited the growth of this opportunistic yeast at concentrations of 25 to 0.2 ,ug/ml (47 to 0.4 ,uM). At these lower concentrations, 2,3,5-triphenyl tetrazolium chloride, an electron acceptor, was reduced by several strains of C. albicans. This effect resulted in red coloration of colonies. Concomitantly, this phenomenon was not antagonized in the presence of ergosterol. Furthermore, neither ketoconazole nor antimycin A inhibited the growth of C. albicans under anaerobic conditions, as revealed by a paper disk method. Ketoconazole at the concentrations stated above inhibited endogenous and exogenous respiration immediately after it was added to a system containing log phase C. albicans cells, as determined polarographically. At the same time, ketoconazole inhibited the activity of NADH oxidase at the mitochondrial level. In contrast, higher concentrations of ketoconazole (>100 ,uM) were required to inhibit the activity of succinate oxidase from rat liver mitochondria. In addition, concentrations of ketoconazole greater than 100 ,uM were required to impair the uptake of labeled leucine and adenine and, subsequently, the incorporation of the former into protein and the latter into DNA and RNA in intact cells. On the other hand, ketoconazole at concentrations of 10, 1.0, and 0.4 ,uM had no effect on either membrane permeability or macromolecular synthesis.Ketoconazole (R41,400) is a new antifungal imidazole compound which has been found to be very active against both superficial and systemic fungal infections when it is administered orally, and its efficacy has been proved in clinical trials. It is known that ketoconazole is much more water soluble and readily absorbed from the gastrointestinal tract than any other chlorinated imidazole derivative synthesized to date. Nevertheless, the excellent in vivo activity of ketoconazole against Candida albicans cannot be explained by its fair blood level after oral administration alone, because the blood level attained is far below the minimal (complete) inhibitory concentration (MIC) in vitro (10).A number of studies have been undertaken to elucidate the mode(s) of action of antifungal imidazole derivatives. The fact that these compounds interact with cell membranes seems to have been well established by the results of Iwata et al. (4,5) and Yamaguchi and Iwata (21,22) with clotrimazole, of with miconazole, and of (3) and by Marriot with some imidazole-containing antifungal agents (9). These interesting findings may provide some clue concerning the primary site of action of these compounds. However, these results do not differentiate the biochemical actions of miconazole and ketoconazole, and the function of the cell membrane components substituted by the precursors of ergosterol in C. albicans is not yet known. Similarly, the majority of these results were obtained after long exposures to the drugs.In ...