Seedling leaves of two pairs of near-isogenic barley lines were inoculated with conidia of the powdery mildew fungus, Erysiphe graminis D.C. f.sp. hordei Marchal, race 3. One set of isolines (RISØ 5678-R and RISØ 5678-S) differed at the Ml-o locus where the recessive allele (ml-o) confers a high degree of race non-specific, penetration-based and papilla-associated resistance to E. graminis, while the dominant allele (Ml-o) allows a proportion of attacking fungal germlings to succeed in infecting. The second isoline set (Algerian-R and Algerian-S) differed at the Ml-a locus where the dominant allele confers race-specific, epidermal cell death resistance visible only by light microscopy. The recessive allele (ml-a) allows a proportion of attacking fungal germlings to succeed in infecting. Leaf samples were taken at 0, 2, 4, 6, 8, 10, 12, 15, 18, 21 and 24 h after inoculation to examine the timing of host epidermal cell cytoplasmic aggregate responses (visible by light microscopy) relative to phenylalanine ammonia-lyase (PAL) mRNA transcript accumulation (determined by quantitative northern blots), and PAL enzyme activity (using radiolabellcd phenylalanine). Erysiphe graminis produced primary germ-tubes (PGTs) within 2 h and appressorial germ-tubes within 6-10 h of inoculation. In all isolines, host epidermal cell cytoplasmic aggregates formed and subsequently dispersed beneath PGTs, between 2 and 10 h, and beneath appressoria between 6 and 15 h. Concurrently, biphasic patterns of PAL transcript accumulation, typified by peaks at 4 and 12 h occurred in all isolines. Temporal patterns of PAL enzyme activity were roughly similar to those of PAL transcript accumulation. Fungal germ-tube contact initiated host epidermal cell cytological responses common to all isolines, induced PAL transcript accumulation, and increased PAL activity regardless of the Mendelian inheritance of "major gene resistance factors" in the barley isoline sets. Thus, there was PAL induction associated with a general defence to infection, but no unusually strong correlation between PAL induction and major gene resistance was found.