A polysaccharide, CSL-0.1, was isolated from the medicinal lichen, Usnea longissima. CSL-0.1 was a neutral rhamnose-containing glucogalactomannan with a molecular weight of 7.86 x 10(4) Da. The polysaccharide had a core mannan structure with (1 --> 6)-alpha-d-Manp units as the main chain and was substituted at the O-2 positions with side chains containing (1 --> 2)-alpha-d-Manp residue, [3)-alpha-Glcp(1 --> 4)-alpha-Glcp(1-->] and 6-O-substituted beta-d-Galf units. 2-O- and 2,3-di-O-substituted Rhap units. The effects of CSL-0.1 on intestinal immunity and antioxidant activity were evaluated. CSL-0.1 increased the spleen and thymus indices in a dose-dependent manner and conferred immunomodulation on reversing the Th1/Th2-related cytokine imbalance in cyclophosphamide (CP)-induced immunosuppressed mice. CSL-0.1 could also enhance the levels of secretory immunoglobulin A in CP-injected mice. Additionally, the antioxidant levels in the liver and intestine of the mice were increased 20%-50% after intragastric injection by CSL-0.1. Animals, Antioxidants/pharmacology, Carbon-13 Magnetic Resonance Spectroscopy, Cytokines/metabolism, Female, Gas Chromatography-Mass Spectrometry, Hydrolysis, *Immunization, Intestines/drug effects, Lichens/*chemistry, Liver/drug effects/metabolism, Methylation, Mice, Inbred ICR, Monosaccharides/analysis, Parmeliaceae/*chemistry, Polysaccharides/isolation & purification/*pharmacology, Proton Magnetic Resonance Spectroscopy, Spectrophotometry, Infrared, Spleen/drug effects, Thymus Gland/drug effects, Antioxidant, Immune system balance, Immunomodulation, Lichen, Polysaccharides, Usnea longissima