Mushroom dietary fiber or nonstarch polysaccharides (NSPs) that were soluble in hot alkali and belonged to the beta-glucan type were isolated from the sclerotia of an edible mushroom, Pleurotus tuber-regium. The mushroom NSPs were further separated into a number of fractions [hot alkali extracts (HAEs)] with weight-average molecular weights (M(w)) ranging from 1 x 10(4) to 42.2 x 10(4). The HAE fractions [with M(w) of (5.8-17.1) x 10(4)] administered intraperitoneally at a dose of 20 mg/kg of body weight to BALB/c mice implanted with solid tumor Sarcoma 180 were found to be effective in inhibiting tumor proliferation with an inhibition ratio of > or =50%. In vitro experiments using human tumor cell lines HL-60 and HepG2 had shown that HAE fractions with M(w) of (5.8-42.2) x 10(4) also had antiproliferative activity at three different concentrations (50, 100, and 200 microg/mL) toward the tumor cell lines tested. All HAE fractions did not inhibit the growth of a normal kidney cell line (Vero) from monkey. It is therefore postulated that the antitumoral effect of NSPs from the sclerotia of P. tuber-regium is probably host-mediated and cytocidal.