The results presented here show that inoculation of barley with wheat powdery mildew (Erysiphe graminis f.sp. tritici), which is non-pathogenic on barley, is followed by an early and rapid activation of a specific set of genes as indicated by the accumulation of specific mRNAs. Two-dimensional polyacrylamide gel electrophoresis of in vitro translation products made from RNA isolated from inoculated and uninoculated barley leaves showed several new polypeptide products at 4 h after inoculation. These products were not apparent at 2 h and they are most probably of plant origin. Induction of new mRNA species correlated with interactions between the primary germ tubes of mildew spores and the epidermis. Analysis of RNA preparations from epidermal strips indicates that at least some of the induced mRNAs are also localized in the epidermis. The results are discussed in relation to other reports of gene expression in barley accompanying resistance responses, and in relation to induced resistance in barley against powdery mildew.