The effects of rice hull (RH) powder on the anticarcinogenic activity of submerged-liquid cultures of Agaricus blazei Murill (AB) were assessed for mouse ascites cancers induced by mouse Sarcoma S-180 (S-180) cancer cells. Optimal growth of AB mycelia in the basal liquid culture medium, containing soybean meal, was achieved by culturing at $25^{circ}C$ for 5 days, when evaluated by $eta$-glucan content, Brix, and mycelial weight, relative to other culture conditions. Hot-water extract (HWE) of the submergedliquid culture of AB mycelia grown at $25^{circ}C$ for 5 days exhibited a stronger anticarcinogenic activity, relative to HWE from other culture conditions. No such effects were obtained from AB mycelial cultures by alternative temperature-controlling cultures. Both cytotoxicity for S-180 cells and anticarcinogenic potentials for mouse ascites cancer of the HWE from AB mycelia grown in the basal medium containing 1% RH powder for 5 days at $25^{circ}C$ were significantly (p<0.05) enhanced, relative to HWE from the AB mycelia culture of the basal medium without RH powder. These results indicate that HWE of submerged-liquid culture of AB mycelia, incubated in media containing 1% RH powder at $25^{circ}C$ for 5 days, enhanced anticarcinogenic activity against S-180 cell-induced mouse ascites cancer, and suggest that RH powder is an excellent ingredient for the improvement of the anticarcinogenic potentials of the submerged-liquid culture of mushroom mycelia.