The therapeutic potential of UK-49,858, a difluorophenyl bis-triazole derivative, has been assessed by evaluating its activity against systemic infections with Candida albicans in normal mice and rats and in mice with impaired defence mechanisms, against vaginal infections with C. albicans in mice, and against dermal infections with Trichophyton mentagrophytes in guinea pigs. Orally administered ketoconazole was used as a comparative agent throughout, and parenterally administered amphotericin B was included in the study of C. albicans systemic infection in normal mice. The activity of UK-49,858 given orally to mice or rats infected systemically with C. albicans was far superior to that of ketoconazole. In addition, UK-49,858 showed activity comparable to that of amphotericin B when given parenterally, although the latter gave more prolonged protection. UK-49,858 was also effective orally in curing experimental candidal vaginitis in mice and trichophytosis in guinea pigs, against which it was approximately 10 times more active than ketoconazole. These data suggest that UK-49,858 may be of value in the treatment of both C. albicans and dermatophyte fungal infections in man.