Pigeon Fever is an equine disease caused by the gram positive bacteria Corynebacterium pseudotuberculosis . The common form of the disease, characterized by external abscesses, is one of the most frequently occurring equine infections in the Western United States. The prevalence of disease is increasing, so a better understanding of the immune response to the bacterial infection along with the development of a vaccine must be considered to protect equine communities. A multi-pronged approach is being used to better characterize the bacteria, the major exotoxin Phospholipase D (PLD), and the equine immune response. To learn more about the bacteria, the 16s rRNA sequences of the equine and ovis biovars of C. pseudotuberculosis were sequenced and compared, and found to be identical. The internal transcribed spacer (ITS) regions of these bacterial genomes will be sequenced next in hopes of finding genetic variation. Phospholipase D is important for the virulence of the disease and is the major bacterial antigen. Purified recombinant PLD is used in tests for anti-PLD antibodies horse sera, and in the development of vaccines. To further study the immune response to C. pseudotuberculosis, a mouse model is being designed and implemented. This model will allow us to determine the effectiveness of different vaccine types against this infection and also provide more insight to the type of cytokine response (TH1 or TH2) to this infection. Gaining a better understanding of the immune response to C. pseudotuberculosis is vital for the ultimate creation of a successful equine vaccine.