Antitumor fraction was purified from dilute alkali extract of leaves of bamboo-grass, Sasa Kurilensis. Conditions were first investigated to obtain reliable data of the effect. Particularly transplantation of suspension of tumor cells diminished the fluctuation of weight of tumors. 10^6/0.2 ml of cell population was necessary for sufficient growth of tumor on the occasion of subcutaneous transplantation. This antitumor fraction contains 71% of lignin. Other components in the preparation, such as 25% of peptides and 3% of sugar which exist combined with lignin do not participate in the activity. It shows no toxicity by subcutaneous or intraperitoneal administration. The fraction was proved to have antitumor activity against NF-sarcoma and sarcoma 180 by the administration of 12 mg per head seven times every other day. In these cases the fraction was administered subcutaneously to the affected part. The activity of liver-catalase of tumor-bearing mice was remarkably recovered by administration of bamboo-grass lignin.