Infestations of cereal crops by Fusarium species have caused severe problems for the North American cereal industries for several years. Fusarium head blight (FHB), or scab, of wheat and barley is caused predominantly by F. graminearum, but other species have also caused epidemics, especially in Europe. The majority of the losses in crop yields and quality from these infestations are probably due to the disruption of the grain structure by the fungal invasion. It is likely that the fungus produces hydrolytic enzymes to break down the grain components in order to obtain nutrients for its growth. We would like to understand how the cereal proteins are degraded by the fungal enzymes. Some fungal proteolytic activity that was associated with changes in storage proteins has been detected in FHB-infected wheat. We have shown that three Fusarium species all produced alkaline proteinases when grown on a medium that contained gluten or autoclaved barley grain. To identify and characterize these enzymes, we have purified two serine proteinases from a F. culmorum culture medium. One had properties like those of chymotrypsin; the other resembled trypsin. We have shown that these proteinases occur in developing barley kernels that have been inoculated with F. graminearum or culmorum spores. In order to localize the enzymes of infected kernels, we have produced antibodies against them in rabbits. We are also isolating and identifying barley proteins that inhibit the activities of these Fusarium proteinases. The interactions between the inhibitors and the proteinases are being studied to clarify whether or not the proteinase inhibitors can be manipulated to improve the resistance of barley to fungal attack.