Preharvest bug damage to wheat can cause significant losses in bread-making quality. One of the most prevalent forms of bug damage which frequently occurs in most countries of the Middle East, Eastern Europe and North Africa can be attributed to Heteropterous insects, particularly Eurygaster spp. Intercultivar differences in the susceptibility of glutenin to proteolytic degradation by the bug Eurygaster integriceps were investigated using six breadwheat cultivars of Turkish origin. Crude enzyme extract was prepared with distilled water from bug-damaged wheat. The freeze-dried extract was blended with sound samples of ground wheat, and the mixture was incubated in distilled water for 30 and 60 min at 37degreesC and subsequently freeze-dried. The proteolytic effects of bug damage were determined on large polymeric glutenin. The latter was measured as 50% I-propanol insoluble (50PI) glutenin extractable with 50% I-propanol in reductant dithiothreitol. The decreases in the amount of 50PI glutenin and the high and low molecular weight subunits were quantified using reversed-phase HPLC. There was a substantial and progressive decrease in the quantity of 50PI glutenin and its subunits with increasing incubation time. Intercultivar differences were observed that were unrelated to intrinsic levels of proteolytic activity. After 60 min of incubation, the relative decrease in 50PI glutenin compared with control samples ranged from 43% (cv. Ankara) to 65 % (cv. Kirkpinar). Some cultivars (Lancer, Ankara and Gun) with similar levels of intrinsic proteolytic activity showed significantly different responses to bug protease. One cultivar (cv. Kirkpinar) with the lowest proteolytic activity was the most susceptible. High quality breadwheats (cvs. Bezostaya, Lancer, Kirac and Gun) were generally more resistant to the bug protease, although Ankara, with both intermediate protease activity and breadmaking quality, was the most resistant cultivar. While the 50PI glutenin test was very effective in quantifying the damaging effects of bug protease on wheat protein quality, the nature of the intercultivar differences was unclear.