To investigate the effects of Paecilomyces cicadae polysaccharide (PCPS) on the immunological function of aged rats in vivo. The young and old rats were administered with normal saline as control groups, and the rats from test group were sc given 50, 100, 200 mg x kg(-1) x d(-1) dosage of PCPS for 3 weeks. The phagocytizing rate and index of PMphi, AMphi to S. aureas were observed, and the colorimetric MTI was used to analyze the proliferative activity of spleenocytes which had been stimulated with ConA or LPS. We also inspected the ability varing of ACP, LDH, ARG of spleen, and observed the ultramicro structure of spleen under the SEM. The phagocytosis of Mphi was lower in aged group than that in young' s group, and the proliferative activity of spleenocytes was lower too. The activities of ACP, LDH, ARG of spleen were extremely decreased (P < 0.01) in aged rats as well. The proliferative activity and phagocytotic rate were both extremely increased in PCPS groups (P < 0.01), and the mitochondrion and endoplasmic reticulum of spleen were accrementition as well (P < 0.01). PCPS could enhance the phagocytizing function of PMphi, AMphi of aged rats in vivo, and strengthen the immune function of spleen and its proliferative activity as well. Then the immunity of aged rats could be improved. The PCPS may be an anti-aging agent.