The autofluorescent cells in the penetration area of powdery mildewed leaves of barley had been reported. The present experiments were performed in order to obtain the fluorescence-inducing extract fraction from the conidia. The preparations were made by extractions and residue which were extracted from the conidia of Erysiphe graminis hordei race I with water, ethanol, and ethyl ether. Bioassaying was carried out on the culled-leaf surface of incompatible Turkey 290 and compatible Kobingataki varieties by placing the drops of extract solutions. Fluorescence-elicitor activity was shown only in tile ethanol-extract fraction to both varieties. However, fluorescence-eliciting rate was more rapid on the leaves of incompatible variety Turkey 290 than on tile those of compatible variety Kobingataki; Turkey 290 less than 8 hours, Kobingataki less than 16 hours.