As a part of the studies on unknown nature of food materials and their use Murakami and Yamafuji (1967) demonstrated an anticancer effect of lignin prepared from bamboo leaves. We have now prepared a polysaccharide and examined its antitumor action. We first established a new procedure for preparation of polysaccharide; alkaline extraction of bamboo leaves was acidified and the supernatant was appropriately treated with ethanol. The sediment was dissolved in water and dialyzed. Light yellow amorphous substance was finally obtained from the dialysate after some treatments. The material was tested for its antitumor effect to solid form of Sarcoma 180. On the 3rd day after tumor transplantation, 0.2 ml of 2% aqueous solution of the material was injected to each of ten test mice (=4 mg/mouse), subcutenously at axilla where the tumor had been transplanted. The control mice were treated with Ringer's solution. Both groups of the animals were treated once every other day and the treatment was continued for 4 weeks. On the 30th day, the tumor were excised, weighed and compared with those of the control mice. In this way the polysaccharide was found to have a notable antitumor effect to Sarcoma 180. Analyses showed that the polysaccharide contains no nitrogen. It consisted of 83% pentose and 15% aldohexose. Neither ketohexose nor uronic acid were tested. The preparation contained 3.5% lignin which may not participate in its antitumor activity. By paper chromatographic methods, xylose, arabinose and galactose were detected as sugar components.