In recent years, a number of studies have been done on the physiological effects of water-soluble polysaccharides (WSPS) and their antitumor and immunoenhancing properties. Many edible mushrooms, in particular those rich in WSPS, not only taste good but also contain ingredients beneficial to the physiology of the human body. In this study, response surface methodology was used to determine the optimal conditions for the production of WSPS, including the C/N ratio, initial pH, and incubation temperature. The highest yield of WSPS was obtained by incubation with a C/N ratio of 40, initial pH 5.5, and an incubation temperature of 25 degrees C. WSPS were extracted by alcohol precipitation from the fermented broth of edible Pleurotus citrinopileatus. These extracts, referred to as SPPC in this paper, had a molecular mass of more than 10(5) Da and were largely made up of glucose and mannose. SPPC was fed to mice which had artificial pulmonary metastatic tumors. Changes in the percentage of the numbers of tumor cells and immune cells were determined by flow cytometry. Daily feeding of SPPC at a dosage of 50 mg/kg to tumor-bearing mice for 12 days resulted in a significant increase in the number of T cells, CD4(+) cells, CD8(+) cells, and macrophages, compared with mice that were not fed any SPPC. The proliferation rate of the pulmonary sarcoma lesions slowed down.