Sun, Z.H., Tang, Z.R., Yin, Y.L., Huang, R.L., Li, T.J., Tang, S.X. and Tan, Z.L. 2009. Effect of dietary supplementation of galacto-mannan-oligosaccharides and chitosan on performance and serum immune parameters of 28-day weaned piglets challenged with pathogenic E. coli. J. Appl. Anim. Res., 36: 207–211. To determine the effects of galacto-mannan-oligosaccharides (GMOS) and chitosan (COS) on growth performance and serum immune parameters 40- 28-d weaned castrated Duroc × Landrace × Yorkshire piglets (8.09 ± 1.87 kg) were challenged with three kinds of pathogenic E. coli (serotype 09, 0141 and 064) and randomly assigned to four treatment groups fed a maize-soybean meal diets containing either no addition (control), 250 mg/kg of lincomycin (LIN), 250 mg/kg of COS (COS) or 2000 mg/kg of GMOS (GMOS), respectively, in α four-week feeding experiment. The feed/gain of control group was poorer than that of other groups with no difference in average daily gain or feed intake. The serum concentrations oflgG, IgA, IgM, IL—6, IL −2 and IL—lβ in experimental groups were higher when compared to control group (P<0.05). The results indicate that GMOS and COS supplemented in weaned piglets diet can improve growth performance and enhance immune function.