Zhang, X., Ding, Y., Qu, M. and Lu, D. 2014. Beneficial effects of ruminal oligosaccharide administration on immunologic system function in sheep. Can. J. Anim. Sci. 94: 679-684. Oligosaccharides (Os) are considered prebiotics with potential beneficial immunomodulating effects, although little is known about their effects in ruminants. Therefore, the objective of this study was to investigate the effects of Os administration on sheep immunologic system function. Sheep were randomly allotted into three different treatment groups (three sheep each): (1) control, infused with equal volume of vehicle distilled water; (2) medium level, infused with 5.4 g Os d^-1 (0.6% of basal diet); (3) higher level group, infused with 10.8 g Os d^-1 (1.2% of basal diet). The study comprised three experimental stages, each with a 14-d adaptation period and a 21-d experimental period; Os were administered during each experimental period via cannulae placed in the rumen, duodenum or ileum, respectively. At 7, 14 and 21 d of infusion, the proportion of circulating CD4 and CD8 T-lymphocytes was assessed via flow cytometry, and serum IgA/IgG concentrations were analyzed using an ELISA test. The Os infusion increased (P<0.05) the proportion of CD4 T-lymphocytes hence raising the CD4 /CD8 ratio, as well the serum concentrations of IgA and IgG. Interestingly, ruminal infusion yielded the best results, while ileal administration resulted in negative measurements. These findings may be useful to elucidate the cellular and molecular mechanisms responsible for enhanced immune functions in sheep receiving dietary supplementation with indigestible oligosaccharides.