Porcine atrophic rhinitis associated with Bordetella bronchiseptica is characterized by a severe inflammatory response in the mucosa of the nasal turbinates. Initial infiltrates of polymorphonuclear leukocytes (PMN) are followed by accumulations of mononuclear cells. In this report, we have investigated the interaction between porcine PMN and B. bronchiseptica. PMN incubated in PBS with a fluorescently labeled hemagglutinating porcine isolate, but not a non-hemagglutinating variant, had high levels of cell-associated fluorescence as determined by flow cytometry. Light microscopy indicated that most cell-associated bacteria were ingested. Transmission electron microscopy confirmed the presence of intracellular bacteria, which were contained within membrane-bound phagosomes. A monoclonal antibody specific for the leukocyte integrin polypeptide CD18 partially inhibited attachment of B. bronchiseptica to normal PMN but not to PMN genetically deficient in CD11/CD18 integrins. Higher levels of inhibition occurred when bacteria and normal PMN were co-incubated in the presence of galactose, N-acetyl-D-galactosamine, N-acetyl-D-glucosamine, mannose and methyl α-D-mannopyranoside. D-glucose, L-fucose, α-lactose and sialic acid had no inhibitory effect. We conclude that B. bronchiseptica is readily ingested by porcine PMN in the absence of complement and antibody and that internalization is mediated by multiple adhesion mechanisms, including CD18- and carbohydrate-dependent ones.