$Elfvingia$ $applanata$, a medicinal mushroom belonging to Basidiomycota, has been used in the effort to cure cancers of the esophagus and stomach, and is also known to have inhibitory effects on hepatitis B virus infection. The hot water soluble fraction (as Fr. HW) was extracted from fruiting bodies of the mushroom. $In$ $vitro$ cytotoxicity tests showed that hot water extract was not cytotoxic against cancer cell lines such as Sarcoma 180, HT-29, HepG2, and TR at concentrations of 10-2,000 ${mu}g/mL$. Intraperitoneal injection with Fr. HW resulted in a life prolongation effect of 45.2% in mice previously inoculated with Sarcoma 180. Treatment of Fr. HW resulted in a 2.53-fold increase in the numbers of murine spleen cells at a concentration of 50 ${mu}g/mL$, compared with control. Incubation of murine spleen cells with Fr. HW at a concentration of 500 ${mu}g/mL$ resulted in improved immune-potwntiating activity of B lymphocytes through an 8.3-folds increase in alkaline phosphatase activity, compared with control. Fr. HW generated 12.5 ${mu}M$ of nitric oxide (NO) when cultured with RAW 264.7, a mouse macrophage cell line, at the concentration of 50 ${mu}g/mL$, while lipopolysaccharide, a positive control, produced 15.2 ${mu}M$ of NO. Therefore, the results suggested that antitumor activities of Fr. HW from $E.$ $applanata$ might, in part, be due to host mediated immunostimulating activity.