Abstract: The present invention relates to preparation of protein polysaccharide 0041 obtained by extracting a dried product of cultured mycelia of Agaricus blazei Murrill 0041 with hot water, and protein polysaccharide 0041 shows immunopotentiating antitumor activity when administered, so that it is useful as an antitumor agent against various tumors, and as an immunopotentiating substance against other diseases.