Lentinan manifests marked antitumor and antimetastatic activity in numerous tumor/host systems, and prevents chemical and viral carcinogenesis. Modulation of immune or vascular functions by lentinan is involved in its antitumor effects. The impact of lentinan on the functions of macrophages is distinct from that of LPS. One of the effects of lentinan on the vascular system is the vascular dilatation and hemorrhage (VDH) reaction, and the effect can be monitored as augmented skin reactions to vasoactive mediators. Lentinan induces the VDH-like reaction at the tumor site, resulting in the induction of hemorrhagic necrosis and complete regression of the tumor. In contrast to LPS-induced tumor necrosis (Shwartzman's-like reaction), lentinan-induced tumor necrosis is T-cell dependent.