Corynebacterium pseudotuberculosis is a bacterium that causes a disease called ?pigeon fever? or ?dryland distemper? in horses. Pigeon fever is endemic in the Southwest. C. pseudotuberculosis survives in the soil for long periods of time, making this one of the most common and economically devastating equine diseases in that area. Since it seems impossible to eradicate the infection causing bacteria from the soil, prevention is the effective way to deal with this disease. Understanding the equine immune response to C. pseudotuberculosis is essential for developing field tests for the early detection and treatment of the disease, and eventually a vaccine. We used Enzyme-Linked ImmunoSorbent Assays (ELISAs) to detect the levels of IgG and IgM antibodies in the serum of infected and control horses against the exotoxin produced by C. pseudotuberculosis, phospholipase D (PLD). Our data showed that there is no major differentiation between the anti-PLD anti-body levels in horses with internal and external abscesses. We also found that some control horses on the same property as infected horses developed antibodies against PLD without developing the disease. The next step in the project is finding out whether these horses have a TH1 or a TH2 immune response to C. pseudotuberculosis by stimulating white blood cell samples and testing their cytokine levels. With this knowledge a vaccine can be made that is directed towards either TH1 or TH2 cells.