Justification - Endotoxemia is an important contributor to mortality and loss of use in the horse and results in significant losses to the equine industry on an annual basis. Objective - To determine the effect of PGG-Glucan on the cytokine response to endotoxin in the horse. Animals - Part 1; 6 adult horses. Part 2; 12 adult horses. Procedure - Part 1; Whole blood was collected, aliquoted, and incubated in vitro in four groups; saline control, endotoxin (LPS) (100 ng/ml), PGG-Glucan (0.1, 1.0, 10 and 100 μg/ml) and LPS (100 ng/ml) plus PGG-Glucan (0.1, 1.0, 10 and 100 μgg/ml). Supernatants were collected at 0, 6 and 12 hours and assayed for tumor necrosis factor £\ (TNF£\) activity. Part 2; Horses received either PGG-Glucan (1 mg/kg) or an equal volume of isotonic saline (0.9% NaCl) IV over 15 minutes. Twenty four hours later blood was collected and mononuclear cells isolated for cell culture. Cells were treated with LPS (100 ng/ml) and RNA extractions were performed at 0, 6, 12, 24 and 48 hours. Relative mRNA expression of TNFα, interleukin-1β (IL-1β),, interleukin-10 (IL-10) and interferon-γ (IFN-γ) was determined by reverse transcription and real time polymerase chain reaction. Results - Using an in vitro endotoxin challenge method PGG-Glucan altered the production of TNFα in a dose-dependent manner. PGG-Glucan had no effect upon the ex vivo cytokine mRNA expression of TNFα, IL-1β, IL-10 or IFN-γ. Conclusions and Relevance - Although mild changes were observed in TNFα production in vitro, it is not likely that PGG-Glucan will have a significant effect upon clinical endotoxemia. / Master of Science