Abstract: OBJECTIVE To investigate the effects of Nostoc sphaeroides Küte polysaccharide on immune function of the immunodeficiency mice. METHODS BALB/c mice were divided into control group, model group, low, medium and high-dose of Nostoc sphaeroides Küte polysaccharide groups(0.1，0.2，0.4 g·kg^-1) and positive drug group(Yupingfeng granules 2.5 g·kg^-1). Animals were given intragastric administration for 28 d. On the 29th day, the rats in other groups were injected with Candida albicans(5×10⁴ CFU) in the tail vein except for the control group, which resulted in an immunodeficiency model. Observations of the weight, organ index, blood routine, spleen morphology, spleen lymphocyte proliferation, peritoneal macrophage phagocytosis and apoptosis, Bcl-2 and Bax protein expression were carried out on the 14th day after injection of Candida albicans. RESULTS Compared with the control group, the rats in model group lost weight, while white blood cells was decreased, neutrophilic granulocytes was increased, thymus and spleen index were decreased, spleen morphology was injured, the proliferation of spleen lymphocytes was decreased, the phagocytic ability of macrophages was decreased, the apoptosis of peritoneal macrophages was increased, Bcl-2/Bax was decreased, all P<0.01. Compared with the model group, the thymus index and the spleen index of rats in all dose groups of Nostoc sphaeroides Küte polysaccharide was increased significantly(P<0.01), while neutrophilic granulocytes was decreased(P<0.01), the spleen morphology was repaired, the proliferation of spleen lymphocytes was increased significantly(P<0.05 or P<0.01), the phagocytosis of peritoneal macrophages was increased significantly(P<0.01), the apoptosis of peritoneal macrophages was decreased significantly(P<0.01), Bcl-2/Bax was increased (P<0.01). CONCLUSION Nostoc sphaeroides Küte polysaccharide can significantly enhance the immune function of immunocompromised mice.