An agglutination test with the use of formalin-killed antigen of the cell carrying the capsule was developed and used for the detection of antibody in swine naturally infected with Bordetella bronchiseptica. Under optimum antigen concentration and reaction temperature 210 or 60% of 342 serum samples tested from 42 conventional swine herds were positive for Bordetella infection. In contrast, only 34 or 10% of 342 nasal swabs from the same animals were positive for Bordetella by culture technique. The test was relatively free of cross-reactivity to related organism. However, 2.7 and 13.0% of sera from growing pigs and mature hogs, respectively, reacted with antigen of Pasteurella multocida. Because of this, only agglutinin reactions in 1:20 dilutions or higher to Bordetella were considered positive. The bulk of the antibody activity of selected sera tested from various age ranges of swine was mercaptoethanol sensitive, suggesting that serum antibody in Bordetella infection may be associated with immunoglobulin IgM. Because of the high agglutinability and stability of formalin-killed antigen the test may be useful as an auxiliary aid for the diagnosis of Bordetella infection where the organism cannot be identified by culture means.