Allicin was effective in vitro against Candida, Cryptococcus, Trichophyton, Epidermophyton, and Microsporum. The minimal inhibitory concentrations (MICs) of allicin against these organisms were 3.13 to 6.25 mug/ml by the agar dilution method and 1.57 to 6.25 mug/ml by the broth dilution method, using Sabouraud glucose (SG) medium. However decreased activity was demonstrated against Aspergillus. The MIC of allicin against various pathogenic fungi was affected considerably by differences in the experimental conditions, e.g., incubation time, inoculum size, type of medium, and medium pH. The MIC of allicin against Candida, Cryptococcus, and Aspergillus remained constant after more than 3 days of incubation, and that against Dermatophytes remained constant after more than 10 days of incubation. Decreasing the inoculum size increased the susceptibility to allicin. The antifungal activity of allicin was stronger on SG agar medium with a pH of 5.6 than on the same medium with a pH of 6.0 or higher. By microscopical observation, allicin induced morphological abnormalities in hyphae of Trichophyton mentagrophytes Morita. Percent germination of spores of the Morita strain at 24 h in SG agar medium was greatly decreased with an allicin concentration of 3.13 mug/ml, and the lethal dose for the spores was about four times higher than the fungistatic concentration. These results suggest that allicin inhibits both germination of spores and growth of hyphae.