A study on character and content of antigen recognized by McAbGB₂ against serum antigen of human breast cancer was reported.The results demonstrated that the antigen recognized by McAbGB₂ is a complex protein which is composed of carbohydrate and protein and it is susceptible to heat. Competitive ELISA test showed that the binding of McAbGB₂ to its antigen could not be inhibited by ferritin and CEA. Western blot revealed that McAbGB₂ recognized antigen with molecular weight of 116 and 45 ku. The antigen recognized by McAbGB₂ exists in breast cancer tissue and serum. ELISA test with McAbGB₂ showed the rate of accord with positivity is 98 %(50/51) in breast cancer patient sera and the rates of false positivity are 6 % (3/50) in normal serum samples tend 6.7% (1/5)in benign breast disease sera respectively. The antigen recognized by McAbGB₂ may be a new tumor-associated antigen.