The antitumor activity of a highly branched (1----3)-beta-D-glucan, SSG, purified from the liquid culture filtrate of Sclerotinia sclerotiorum IFO 9395 and its several derivatives were treated in ICR mice bearing Sarcoma 180 cells. SSG was effective by both systemic (intraperitoneal and intravenous) and local (intratumoral) administrations on the solid form of Sarcoma 180 in ICR mice and the mice acquired resistance to subsequent inoculation of Sarcoma 180. However, SSG was not effective on the ascites form Sarcoma 180. The pretreatment of ICR mice with carrageenan suppressed the antitumor activity, suggesting the involvement of macrophages on the antitumor activity. Derivatives prepared from SSG by periodate oxidation/borohydride reduction showed antitumor activity, but those obtained after acetylation, carboxymethylation and hydroxyethylation were less active. From these results, it is suggested that SSG is a useful antitumor glucan which modifies biological responses and can be used as a source for some antitumor derivatives.