A preliminary study was made on the characterization and distribution in human tissues of AF9-recognized antigen. The results demonstrated that the antigen is a complex protein composed of carbohydrate,lipid and protein and susceptible to heating to 100℃.The determinant of AF9-recognized antigen was not present in the ferritin and carcinoembryonic antigen. Western blotting revealed that AF9-recognized antigen with molecular weights of 51,56,67 and 73 kD.Immunohistochemical staining showed that the antigen mainly existed in the cytoplasm and on the membrane of breast cancer cell, and also could be observed in some other tumors, but no staining was detected in normal tissues. The AF9-recognized antigen may be a new tumor-associated antigen.