The antitumor effect of reserve polysaccharide, paramylon, from Euglena gracilis on the transplantable sarcoma-180 was examined in mice. This polysaccharide had an effect similar to that of lentinan. Paramylon, in a dose of 1 mug/g body weight, injected intraperitoneally 24 hr after tumor implantation had an inhibitory effect on the tumor growth, although without causing complete regression of the tumor. Alkaline-treated paramylon had a similar effect but at a smaller concentration than the native one. The inhibitory activity was not lost when the paramylon preparation was treated with pronase, DNase, or RNase. The antitumor effect might be a lymphocyte-mediated process. In tumors that were regressing after treatment, there was extensive outpouring of lymphoid cells with plasma cells and macrophages. A test conducted using paramylon ruled out the possibility of an interferon-mediated inhibiotry effect on tumor growth.