The proteolytic activity of the leaf extracellular space of wheat cultivars Pigüé and Isla Verde was estimated after inoculation of either detached leaves or plants with the fungusSeptoria tritici. Pigüé is resistant, whereas Isla Verde is susceptible to the disease caused byS. tritici. Viable conidiospores of the fungus caused similar increases in both hydrogen peroxide production and chitinase activity of the cultivars studied. In contrast, they caused a decrease in the extracellular serine proteinase activity of Isla Verde and a significant increase in that of Pigüé. Independently of the cultivar from which it was extracted, the extracellular serine proteinase inhibited the germination ofSeptoria triticiconidiospores. These results suggest that the proteolytic activity of the leaf extracellular space can participate in the defence of wheat plants againstSeptoria tritici. Its regulation may be controlled by specific defence components of each cultivar.