Equine Protozoal Myeloencephalitis is the most frequently diagnosed neurologic disorder of horses in the united states, which is caused by the protozoan organism Sarcocystis neurona. The disease has a profound impact on the American Horse Industry. This impact includes prolonged and expensive treatment without a guaranteed return to a previous level of use for the individual horse. Poor respponse to and prolonged duration of treatment may suggest an immune mediated impariement of host response. There is limited information about the direct interaction between the pathogen and the host. In two in vitro experiments we investigated a) whether the presence of the protozoan organism can influence mitogen-stimulated peripheral blood mononuclear cells (PBMCs), suggesting a direct influence of the protozoan organism on cells of the immune system, and b) if cerebrospinal fluid (CSF) from horses with EPM has an effect on mitogen-stimulated PBMCs, suggesting that the microenvironment of the site of infection influences the course of disease. Experiment 1: Mitogen simulated PBMCs from EPM affected and control horses were co-cultured with fragments of freeze thawed bovine turbinate cells that were infected with S. neurona merozoites. Compared to controls PBMCs co-cultured with S. neurona fragments were the only cells that showed a decreased proliferation (p<0.05). A difference between EPM affected and control horses could not be detected (p>0.05). These results may imply that the persistence of S. neurona infection in the horses CNS is, in part, due to a pathogen-derived mechanism that attentuates the hosts immune response. Experiment 2: Mitogen stimulated PBMCs from a horse affected with EPM and a control were co-cultured n the presence of CSF from EPM affected and uninfected controls. Prior to co-culture the CSF was fractionated by a filtration process over two microfilter units. An identical volume of NaCl (0.9%) served as a control for the volume of CSF that was added. The proliferation assay revealed a deviation of the response depending on cell donor and CSF fraction used. The effect was independant of the protein concentration of the CSF fraction, and a decrease in lymphocyte proliferation was not caused by increased cellular death. This suggests the presence of subsets within the CSF which have a stimulatory of suppressive influence on the cells in culture. The effect was cell donor dependant which implies a difference in lymphocyte subsets between the two horses that were used. / Master of Science