Serogroup A strains of Pasteurella multocida, the major cause of fowl cholera, are resistant to phagocytosis in nonimmunized birds. Adherence studies with a capsulated strain of P multocida (serotype A:3) and turkey air sac macrophages in culture showed that the bacteria were capable of adhering in large numbers to the macrophages but were not internalized. A noncapsulated variant of the bacteria (serotype -:3) showed little or no adherence and was not internalized. These data indicated that the adhesive properties were caused by the presence of a capsule on the bacteria. The role of capsular hyaluronic acid in adherence to macrophages was investigated. Depolymerization of the bacterial capsule with hyaluronidase increased phagocytosis by macrophage cultures, and addition of hyaluronic acid to the macrophages inhibited bacterial adherence. Additionally, exposure of macrophages to chondroitin sulfate B, an anionic polysaccharide similar to hyaluronic acid, did not affect the adhesive properties and resistance to phagocytosis of capsulated organisms. Treatment of macrophages with sodium metaperiodate or trypsin suppressed bacterial binding. Collectively, these data indicate that ?P multocida adhesion to air sac macrophages, but not internalization, is mediated by capsular hyaluronic acid and suggest that recognition of this bacterial polysaccharide is a result of a specific glycoprotein receptor