A possible mechanism for resistance has been observed in powdery mildewed barley (Erysiphe graminis f. sp. hordei on Hordeum vulgare). A substance which stains with basic protein staining procedures occurs in papillae of susceptible and resistant powdery mildew infected barley. It is first detectable after the infection peg penetrates the host cell wall. In susceptible host cells this substance is confined to the papilla, whereas in resistant host cells this substance spreads throughout the penetrated cell and to adjacent mesophyll cells beneath the infected cell. The spread of this substance correlates with the expression of resistance in the host cells.