The antitumor effect of exo-biopolymer (EXP) produced by Ganoderma applanatum was investigated using sarcoma-180 bearing mice. EXP, when administered (10-80 mg/kg body weight: BW) intraperitoneally, significantly inhibited the growth of solid tumor and increased the natural killer (NK) cell activity. A dose of 40 mg/kg BW was found to be highly effective, as it reduced the tumor formation by 39.7%, and increased the NK cell activity of splenocytes by 51.6% compared with the control group. The complement activity of EXP was increased in accordance with an increase in concentration. The phosphatase activity of macrophages was increased by 0.7-fold (200 microg/mL) compared with the control group. This EXP contained 58.9% carbohydrate and 17.1% protein. The major sugar of EXP was composed of mannose and glucose, while the protein mainly consisted of serine, glycine and aspartic acid.