Seedling leaves of two barley lines, Algerian/4*(f14) Man (S) and RISØ 5678-S, were excised and supplied either with water (controls), the phenylalanine ammonia lyase inhibitors AOA (α-aminooxy acetic acid) or AOPP (α-aminooxy-β-phenylpropionic acid), or the cinnamyl alcohol dehydrogenase inhibitor OH-PAS ([[(2-hydroxyphenyl)amino]sulphinyl]acetic acid, 1,1-di-methylethyl ester) by immersing their cut ends in solution. After 24 h leaves were inoculated with compatible Erysiphe graminis f.sp. hordei conidia and returned to the test solution for 36 h incubation. Treatment with AOA had no effect on the percentage of conidial germlings which penetrated the barleys successfully (formed haustoria), nor on the occurrence of localized autofluorescent host cell responses associated with fungal primary germ-tubes or appressoria. However, both AOPP and OH-PAS treatment led to increased quantitative susceptibility of both barley lines as the percentage of appressoria penetrating host cells to form haustoria was approximately doubled. This was associated with reduction in both the frequency and intensity of autoflu orescent responses associated with fungal germ-tubes; in this respect, the effects of AOPP were slightly more pronounced than those of OH-PAS. The results support the view that in barley, as in oats, autofluorogenic compounds accumulating at sites of germ-tube contact with host cells are phenolic compound(s) synthesized de novo following pathogen attack, that these compound(s) are products synthesized as part of the lignin biosynthetic pathway, and that the compound(s) are involved with resistance to attempted penetration from appressoria.