The weaning process of foals involves a period of considerable stress which likely contributes to an increased risk of infectious disease in these young horses. Mechanisms responsible for this heightened risk of infection remain unknown, although likely due to compromised cell-mediated immunity. Parapoxvirus ovis (PPVO), an immmunomodulator, has been shown to limit the severity of infectious disease outbreaks among horses and has been shown to enhance CMI responses. Thus, an objective of this study was to investigate the effect of PPVO therapy on cell-mediated immune (CMI) responses of abruptly weaned foals. A group of foals (n=6) were given an intramuscular injection of PPVO on days -2, 0 (weaning) and 9. An additional group of foals (n=5) received the diluent only on the same days serving as controls. Peripheral blood samples were collected from all foals prior to weaning (day 0) and on days 1, 3, 5, 7, 9, 11, 14, and 21 after weaning. Whole blood samples were prepared to determine in vivo cytokine mRNA expression by reverse transcription and real-time PCR (RT-PCR). Peripheral blood mononuclear cells (PBMC) were isolated and stimulated to determine in vitro cytokine production by intracellular staining using flow cytometry and gene expression was measured by RT-PCR. Cytokines analyzed in this study were interferon-gamma (IFN-γ), tumor necrosis factor-alpha (TNF-α) and interleukin-10 (IL-10). Regardless of PPVO treatment, foals undergoing the weaning process showed a significant decrease in both in vivo and in vitro cytokine (IFN-γ, TNF-α and IL-10) production. These results indicate that abrupt weaning significantly impacts CMI of the foal which may increase susceptibility to infectious agents.