Antitumor effect of endo-biopolymer (EBP) obtained from submerged mycelial culture of Pleurotus eryngii was investigated by using Sarcoma-180 bearing mice. The administration of EBP, intraperitoneally (10 to 80 mg/kg BW) at all dose ranges, inhibited the growth of the solid tumor and simultaneously increased the activity of natural killer (NK) cells significantly. While, 40 mg/kg BW treated group reduced the tumor formation (53.1%), and increased the NK cell activity of splenocytes (25.0%) to a great extent. The complement activity of the EBP was 43.1% ($1,000;{mu}g/mL$). Phosphatase activity of macrophage was found to be increased up to 35.3% ($100;{mu}g/mL$) when compared with control. The chemical composition of EBP was found to contain carbohydrate (77.5%) and protein (21.6%). The major component sugars of EBP were glucose and mannose, while glycine, glutamic acid, and aspartic acid mainly constituted the protein moiety. These results revealed that EBP obtained from submerged culture of P. eryngii stimulate not only immune system but also antitumor activity.