The effects of cylindan, a polysaccharide isolated from the basidiocarps of Agrocybe cylindracea, on murine sarcoma 180 tumor and murine immune cells were examined after intraperitoneal administration. Cylindan exhibited a marked life extension effect in mice against ascite forms of sarcoma 180 and Lewis lung carcinoma at a dose of 50 mg/kg/day, although it did not show any direct cytotoxicity against sarcoma 180, X5563, and MM46 murine tumor cells. Cylindan increased numbers of bone marrow stem cells as well as peritoneal exudate cells in flow cytometry using monoclonal antibodies. The tumor bearing mice group apparently showed the increase of macrophages and cytotoxic T lymphocytes in mouse spleen cells during the early stage of tumor growth. But during the later stage, the control group decreased immune cells and cylindan restored the decreased immune cells in the tumor bearing mice to the normal level. In non-specific immune response, cylindan stimulated the bacterial phagocytosis and acid phosphatase production in macrophages. It also activated components of the alternative complement pathway and natural killer activity against YAC-1 lymphoma. In number of plasma cells as token of stimulation of the differentiation of B lymphocytes. In cellular immunity, cylindan restored the depressed response of delayed type hypersensitivity in the tumor bearing mice to 60% of the normal level and increased the interleukin-2 (IL-2) responsiveness in the IL-2 dependent CTLL-2 cells. These results suggest that cylindan did not show direct cytotoxic effects on tumor cells but restored the decreased immune response of the tumor bearing mice.