Corynebacterium pseudotuberculosis is a gram-positive facultative intracellular 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. Most infected horses develop large pus-filled external abscesses, which resolve with a low mortality rate. Some horses develop internal abscesses, with a subsequent mortality rate of about 40%. Our hypothesis, which reflects the current diagnostic criteria, was that horses with internal abscesses will have high levels of IgG anti-PLD antibodies while horses with external abscesses will have low levels of IgG anti-PLD antibodies. We also predicted that IgM anti-PLD antibody levels will not be significant. Through the testing of 67 horses by ELISA we have found that IgM anti-PLD antibody levels are similar in infected and uninfected horses as we predicted, meaning that IgM will not be diagnostically significant. However, our results show that while most horses with internal abscesses show high IgG antibody responses to PLD, many horses with external abscesses and some uninfected horses also have high titers to PLD. This same trend was seen when the ELISA was used to test the sera for IgG antibody subclasses (IgGa, IgGb, IgGc, Ig(T)). This leads us to the conclusion that the difference in disease manifestation is not reflected by the type of antibodies produced. We have also concluded that the current diagnostic criteria need to be revised.