The ABH blood group isoantigen status of a retrospective series of 233 invasive breast carcinomas was examined, employing monoclonal antibodies (MCAB) against A, B, and H antigens with the avidin-biotin-peroxidase complex method. In addition, the H antigen was localized with Ulex Europeus Agglutinin I (UEAI) binding. The MCABs provided consistent and specific staining of erythrocytes and endothelium, as well as normal and neoplastic breast epithelium. The anti-H MCAB yielded cleaner background and less intense staining, but otherwise the staining distribution was comparable to the UEA I technique. Contrary to previous reports, deletion of isoantigen expression was not universal in all invasive carcinomas. Whereas 64%, 77%, and 73% of carcinomas from groups A, B, and AB patients, respectively, demonstrated total isoantigen loss, the remaining tumors exhibited variable degrees of isoantigen expression. Moreover, those carcinomas with complete loss of A and B determinants still displayed variable degrees of H immunoreactivity. Carcinomas from group O patients had different degrees of H antigen deletion, with only 12% showing total loss. Statistical analysis revealed that the isoantigen status bore no significant relationship to various epidemiologic, clinical, and pathologic parameters and did not serve as a useful prognostic determinant.