An international collaborative study was performed in order to propose quality control limits for voriconazole disk diffusion tests on Mueller-Hinton agar with 2% glucose and 0.5 micro g of methylene blue per ml. The supplement may be added to the agar before autoclaving, or Mueller-Hinton agar plates may be flooded with a glucose-methylene blue solution. Replicate tests on both types of agar plates with 1- micro g voriconazole disks generated data to propose zone size limits for tests of Candida parapsilosis ATCC 22019 (28 to 37 mm), Candida albicans ATCC 90028 (31 to 42 mm), and Candida krusei ATCC 6258 (16 to 25 mm). Candida tropicalis ATCC 750 was not useful for this purpose.