A novel method is proposed for the evaluation of the activity of an antifungal agent administered as a gas. This system is composed of a batch-flow type reaction vessel, a gas flow system, and a microscopic observation system. The agar plate was prepared on the ceiling of the reaction vessel, and the mycelium of a fungus (Aspergillus niger or Rhizoctonia solani) was inoculated onto it. After preincubation at 25 degrees C for 24 h, the reaction vessel was connected to the gas flow system. An appropriate hypha was selected, and its elongation rate was measured. Then a sample holder containing an antifungal compound was inserted into the reaction vessel from the side hole to saturate the atmosphere inside with its vapor. The retardation or inhibition of the hypha elongation was observed on a television monitor and recorded on a video tape recorder. The antifungal compound was then removed, and the reaction vessel was flushed with air. If the hypha lived, it began to elongate again. By this method, antifungal activity of seven odor compounds could be evaluated quantitatively within several hours.