Eighteen-day-old turkey poults were inoculated intratracheally with either fimbriated or non-fimbriated strains of E. coli. Lung was examined histologically and ultrastructurally with both transmission electron microscopy and scanning electron microscopy. E. coli 078 was identified using rabbit anti-0 antibody and protein A-labeled with colloidal gold;All E. coli infected poults became bacteremic and there were no significant differences between groups. Both fimbriated and non-fimbriated E. coli adhered to and infected air capillary epithelial cells. Bacteria were within vacuoles in portions of epithelial cells that lined the fornix of aircapillaries. In addition, bacteria were seen in the subjacent basement membrane and, on rare occasions, in vacuoles of endothelial cells. E. coli also infected both granular and nongranular cells that lined air atria and were within trilaminar substance. Bacteria were present between reticular fibers of the interstitial stroma and pleura at 30 minutes post inoculation and thereafter;In a second study, lectin binding characteristics of respiratory epithelia were determined by using biotinylated lectins to identify possible fimbrial adhesin sites in situ, e.g., D-mannose residues. Sialic acid residues were identified using neuraminidase digestion. Colloidal gold-labeled PNA (peanut agglutinin) was used to identify cells bound by this lectin with scanning electron microscopy in the BE (or backscatter mode). One N-acetylglucosamine-binding lectin and two N-acetylgalactosamine-binding lectins stained the apical cytoplasm and membranes of normal and hyperplastic granular cells that lined air atria. Three other lectins in these groups stained atrial cells and also ciliated cells of the trachea and bronchi and air capillary epithelial cells. Apical membranes of epithelial cells lining the trachea, bronchi and air capillaries as well as endothelial cells of arteries and veins expressed sialic acid residues. Three D-mannose-binding lectins stained reticular and elastic fibers of the tracheal and bronchial lamina propria, pulmonary interstitial fibers and the tunica adventitia of arteries and veins but did not stain apical membranes of respiratory epithelial cells.