Using C3H/He mice a series of experiments was carried to study the antitumor effect of lipopolysaccharide (LPS) in combined use with Lentinan extracted from Cortinellus shiitake and its influence on cellular immunity as well as antitumor effect of tumor necrosis factor (TNF) prepared from LPS, and the following results were obtained: 1) In the cases where LPS was administered when the tumor had grown to a certain extent after tumor transplantation, hemorrhagic necrosis of the tumor occurred within 48 hours, showing a high antitumor effect. 2) In the cases where LPS was used with Lentinan, i.e., 2 mg/kg/day of Lentinan was administered from the 1st to the 8th day after tumor transplantation, and 30 micrograms/kg of LPS given on the 12th day, the highest antitumor effect could be achieved. 3) By combined use of LPS and Lentinan delayed type hypersensitivity was intensified. 4) Upon application of rabbit serum containing TNF to cancer bearing mice, hemorrhagic necrosis of the tumor was observed within 48 hours, and a superior antitumor effect compared to LPS alone was noted in terms of tumor size and weight.