The saccharide antigen, Gal beta1-3GalNAc or T antigen, is of biologic importance in many systems. It is a tumor-associated carbohydrate antigen, a temporally expressed antigen in germinal center B cells and cortical T cells, a parasite-associated antigen, a spermatozoa vitality marker and an antigen on aged red blood cells. It may play a role in normal cellular adhesion and in tumor cell metastasis. Well characterized monoclonal antibodies (MAb) to Gal beta1-3GalNAc will be useful for further studies in these areas. We developed an IgG3 MAb to Gal beta1-3GalNAc by immunizing the mice with a synthetic Gal beta1-3GalNAc-BSA conjugate. The MAb was analyzed using inhibition enzyme immunoassays with related synthetically prepared sugars to determine the restrictions involved in the antibody binding. Use of synthetic sugars as competitors enabled us to delineate the epitope restrictions on the binding activity of this monoclonal and will enable use of this MAb in studies concerning the biologic importance of this disaccharide.