Escherichia coli infections are a major problem for the poultry industry in the United States. Yet, the virulence mechanisms operative in avian E. coli are poorly understood. In the present studies, monoclonal antibodies (MAbs) have been generated that may facilitate study of the pathogenesis of avian colibacillosis. These MAbs are directed against the Iss protein because results from our laboratory have shown that the possession of iss DNA sequences is strongly correlated with the E. coli implicated in avian colibacillosis. As part of an overall effort to explore the role of iss/Iss in colibacillosis pathogenesis, Iss protein has been purified, MAbs to Iss have been generated, and the MAbs are being evaluated. B cells from mice immunized with an Iss fusion to glutathione-S-transferase produced antibodies specifically against Iss, and these cells were used to generate the MAbs. These anti-Iss MAbs, when used in western blotting assays, can be used to distinguish iss-positive and -negative E. coli isolates, suggesting that they may be useful as reagents in the detection and study of virulent avian E. coli.