1. Lentinan inhibited the proliferation of MH 134 ascites hepatoma transplanted subcutaneously into C3H/He mice, but its most favorite effect appeared when 1mg-2mg/Kg of lentinan was administered for 10 consecutive days from the eighth day after tumor transplantation, yielding a tumor proliferation-inhibition rate of 33% in the average tumor diameter. 2. In studying the average survival days by the chemotherapy of mitomycin C (MMC), 5-fluorouracil (5-FU), and cytosine arabinoside (Ara-C) in combination with lentinan, when both of these substances were administered concurrently in the second week of the tumor transplantation, average survival time was 29.2 days as compared to 20.5 days in the untreated group, 25.1 days in the group administered lentinan alone, and 22.0 days in the group receiving chemotherapy alone. 3. When the antitumor activity of lentinan was studied by the change in the macrophage migration inhibition activity (MI), it was found that in the untreated group MI activity disappeared on the 14th day after tumor transplantation, while in the group treated with lentinan observation of a positive activity in the spleen cells suggested a restorative action of lentinan of the immunity suppression accompanying the tumor growth.