Seedling leaves of oat cv. Selma, barley cv. Golden Promise and wheat cv. Cerco, were excised and infused either with water or with 10 −3 m α-aminooxy-β-phenylpropionic acid (AOPP), a specific inhibitor of phenylalanine ammonia lyase (PAL). Seedling leaves of the three cereal species from both treatments were inoculated with an isolate of Erysiphe graminis from each of the formae speciales avenae, hordei and tritici. In all appropriate host-parasite combinations, AOPP treatment decreased the percentage of primary germ tubes and appressoria associated with localized autofluorescent host cell responses, and increased the percentage of appressoria forming haustoria i.e. increased the susceptibility of epidermal cells to haustorium formation. Thus, autofluorogens appear to be phenolic compounds associated with resistance to infection of these cereal species by their appropriate forma specialis of E. graminis. AOPP treatment also decreased the percentage of host cells which showed whole-cell autofluorescence, indicating cell death, as a result of attack by an appropriate fungal isolate. In appropriate combinations, whole-cell autofluorescence was most common in the barley f. sp. hordei combination, where decreased epidermal cell death due to AOPP treatment may have contributed to increased rates of haustorium formation. For appropriate combinations this is the first report showing AOPP-associated increase in quantitative susceptibility (reduction in background resistance) for barley and wheat. In inappropriate combinations, AOPP treatment decreased the percentage of primary germ tube contact sites associated with localized autofluorescence, but did not decrease the percentage of appressoria associated with localized autofluorescence. In only one of six inappropriate combinations (oat f. sp. tritici) did AOPP treatment increase rates of haustorium formation above the very low levels seen in all inappropriate host-parasite combinations in water-treated leaves. The failure of AOPP to suppress localized autofluorescent responses to fungal appressoria and to increase susceptibility, in most inappropriate host-parasite combinations suggests that resistance factors operate in inappropriate combinations which are not present in appropriate combinations. Although AOPP treatment decreased the relatively high percentages of dead, autofluorescent cells resulting from inappropriate interactions, decreased cell death was not accompanied by increased haustorium formation i.e. cell susceptibility was not increased.