Polysaccharides play a key role in enhancing immune function and facilitating cellular communication. Here, we purified Nymphaea rubra Roxb. polysaccharides (NR-PS) by treating them with pullulanase. They were then cultured with immature dendritic cells (DCs) derived from rat bone marrow hematopoietic cells (BMHCs). After treatment with bioactive NR-PS with a degree of polymerization (DP) value of 359.8, we found that the DCs underwent morphological changes indicative of activation. CD80/86 (87.16% +/- 8.49%) and MHC class II (52.01% +/- 10.11%) expression levels were significantly up-regulated by this treatment compared to the controls (65.45% +/- 0.97% and 34.87% +/- 1.96%). In parallel, endocytosis was also reduced (167.94% +/- 60.59%) after treatment with 25 mu g/mL of NR-PS as measured by the medium fluorescence intensity compared to the control (261.67% +/- 47.26%). Furthermore, the DCs after treatment with 25 mu g/mL NR-PS showed increased IL-12 (102.09 +/- 10.16 to 258.78 +/- 25.26 pg/mL) and IFN-gamma (11.76 +/- 0.11 to 15.51 +/- 1.66 pg/mL) secretion together with reduced IL-10 secretion (30.75 +/- 3.35 to 15.37 +/- 2.35 pg/mL), which indicates a T(H)1 immune response. In conclusion, NR-PS exhibits stimulatory effects on rat DCs and promotes the secretion of T(H)1 cytokines. Taken together, our studies are the first to show that NR-PS is an immunomodulator affecting the maturation and functioning of DCs.