Conidial proteins from barley powdery mildew, Erysiphe graminis f. sp. hordei, were separated by 2-dimensional electrophoresis in polyacrylamide slab gels. Isoelectric focusing was used in the first dimension and separation according to molecular weight in a gel containing sodium dodecyl sulphate (SDS) in the second dimension. The protein patterns obtained were insensitive to environmental variations, host genotype and age of the conidia. Seven mildew cultures of diverse origin were each characterized by a unique and reproducible pattern involving 174 predominant protein spots. The average coefficient of similarity was 0–95. A wild type culture and a mutant differing only in pathogenicity to barley lines with resistance gene Ml-g, had identical patterns.