A mutant variant of a septicemic Escherichia coli strain (L3) isolated from an outbreak in chickens was constructed by the insertion of TnphoA transposon. Seven mutant derivatives were analyzed regarding the pathogenicity. Two of them (XP2, XP4) were less pathogenic in the one-day-old chick pathogenicity assay. The expression of several outer membrane proteins of mutant XP2 strain was suppressed, and strain XP4 had a 47.8(kDa) protein that was not expressed. None of these proteins was correlated to the iron-acquisition system. Mutant XP2 could have suppression of a regulatory protein responsible for the expression of other proteins not related to pathogenicity but important for the rapid bacterial growth, while mutant XP4 did not express a 47.8(kDa) protein. We propose that the 47.8(kDa) protein could be associated to the pathogenicity process of Escherichia coli strains responsible for septicemia in poultry.292113117