Abstract CaMBr1 is a tissue-specific and tumor-associated saccharidic epitope, defined by mAb MBr1 (Ab1), expressed on glycoconjugates of the human mammary carcinoma cell line MCF-7 and of normal and neoplastic mammary epithelial cells. An anti-anti-idiotypic monoclonal Ab3, 2G-3, identifying a human breast tumor associated antigen, was raised by using as immunogen a mouse anti-idiotypic monoclonal Ab2, A3B10, which behaves as the internal image of CaMBr1. mAb 2G-3, as well as MBr1, defines a saccharidic epitope on glycoconjugates extracted from MCF-7 cells and shows MBr1-like reactivity on normal and neoplastic-tissues. Experimental evidence, however, suggests that the fine immunoreactivity of the two antibodies is not identical, because MBr1 has a preferential reactivity with glycolipids and 2G-3 with glycoproteins. We suggest that a possible biologic explanation for our findings could reside in the nature of the immunogens used to raise the two mAb (glycolipid vs protein "internal image").