Six patients with culturally proven Pasteurella multocida infection were evaluated serologically. The infections were 1 foot abscess, 1 septicemia, 3 bronchitis and 1 bronchopneumonia. Most of them were elderly women closely exposed to pets or domestic animals. The serotypes of the strains were determined in 5 cases (3 A3, 2 A7). Specific antibodies against capsular and somatic antigens of P. multocida were determined by indirect hemagglutination and agglutination respectively. The antibodies were strictly directed against the capsular and somatic specificities of the isolated strain. The range of the serum antibody titers were 20 to 2,560 to capsular antigens and 5 to 640 to somatic antigens within 2 weeks after the first clinical signs of infection. Several months after successful treatment, the capsular antibodies were lower while the somatic antibodies had almost disappeared. These findings suggest a good sensitivity for these serologic methods in active cases. Declining antibody titers follow healing. Cross-reactivity of the serologic tests with other bacteria was not observed. Serologic diagnosis of P. multocida infections is a possible alternative to direct diagnosis when cultures are negative or when unusual localizations must be confirmed.