Preincubation of bovine neutrophils with Pasteurella haemolytica A1 purified capsular polysaccharide markedly diminished their ability to ingest and kill P. haemolytica, but not Escherichia coli, in vitro. Ingestion and killing were impaired even when the P. haemolytica were preopsonized, thus suggesting that the inhibitory effects of the polysaccharide included a direct effect on bovine neutrophils rather than simply competition for serum opsonins. Preincubation of neutrophils with purified polysaccharide did not elicit a chemiluminescence response, nor did it alter the chemiluminescence response of neutrophils to subsequent stimulation with opsonized P. haemolytica or opsonized zymosan. In addition, purified polysaccharide alone was neither chemotactic nor did it induce a shape change in bovine neutrophils. These data suggest that the deposition of capsular polysaccharide in the lung during the onset of pulmonary pasteurellosis might impair the ability of neutrophils to ingest and kill P. haemolytica. The capsular polysaccharide of P. haemolytica, therefore, may contribute in part to the fibrinous pleuropneumonia that characterizes acute pasteurellosis.