Phospholipase activity, probably C-type, increased when cultivars of wheat were infected in incompatible, but not in compatible, combinations with wheat stem rust (Puccinia graminis Pers. f. sp. tritici Eriks. & Henn) or with the non-pathogenic crown rust of barley (P. coronata Cda. f. sp. avenae Fraser & Led.)- The effect was mimicked when an elicitor fraction isolated from germ tubes of the wheat stem rust was injected into leaves. The increase in phospholipase activity correlated with the hypersensitive response of leaves when infected with incompatible rust fungi or treated with fungal elicitor. This effect was not observed in healthy or in water-injected control plants. The results suggest that membrane degradation is involved in the hypersensitive response.