Biochemical studies have revealed the alteration of carbohydrate structures of cell membrane glycolipids, glycoproteins and cell secretory products associated with neoplastic transformation. Immunohistochemical examination of the blood group carbohydrate antigens in various cancer tissues have revealed the following results. 1) Incomplete synthesis of carbohydrate chains (e.g. loss of ABO antigens) 2) accumulation of precursor carbohydrates (e.g. accumulation of I antigen which is one of the precursors of ABO) 3) synthesis of new carbohydrates (e.g. expression of A-like antigens in cancer of O & B hosts). Many monoclonal antibodies raised against cancer cells have been shown to react with these, or modified, blood group carbohydrates. Monoclonal antibodies NCC-LU-35, or-81 obtained by immunizing with lung cancer reacted with the Tn antigen (GalNAc directly linked to serine or threonine), which is formed by incomplete synthesis of mucin-type carbohydrates including MN blood group antigens. These antibodies against Tn antigen cross-reacted with A glycolipids, since Tn antigen and A glycolipids share terminal GalNAc. Therefore, Tn antigen was concluded to be an A-like antigen in a broad sense. Tn antigen is expressed in many cancers and some hyperplastic lesions but is undetectable in various normal tissues. These studies indicate that alterations of blood group-related carbohydrates may be good markers for diagnosis and treatment of cancers.