The integrins are alpha beta heterodimeric transmembrane proteins mediating cell-substratum as well as cell-cell interactions. To identify the pattern of expression of the beta 1, beta 3, and beta 4 integrins and their accessory adhesion molecules in relation to the malignant phenotype of invasive breast cancer, we performed an immunohistochemical study for the alpha 2 beta 1 (VLA-2), alpha 6 beta 1 (VLA-6), alpha v and alpha v beta 3 (vitronectin receptor), alpha 6 beta 4, carcinoembryonic antigen, and carcinoembryonic antigen-related molecules in a series of 37 invasive breast carcinomas. All integrin chains examined showed similar patterns in nonneoplastic breast tissue, with strong membrane staining of the myoepithelial cells and weak to moderate staining on the basolateral surfaces of the luminal cells. We found that downregulation of the alpha 2 chain of VLA-2 occurs more frequently in poorly differentiated grade III invasive ductal carcinomas (IDCs) (P = .048). Loss of alpha 6 beta 4 seems also to occur more frequently in grade III IDC (seven of 11 cases, 63.6%) than in grade I/II IDC (two of eight cases, 25%), although this did not reach statistical significance. Carcinoembryonic antigen and carcinoembryonic antigen-related antigens, which are known to function as accessory adhesion molecules, were found mainly in the cytoplasm of neoplastic cells and there was reduced membrane polarization in poorly organized tumors. In contrast the alpha v beta 3, vitronectin receptor heterodimer recognized by the 23C6 monoclonal antibody was weak or absent in normal breast epithelium, and was weakly expressed in two of 19 (10%) IDCs and in nine of 18 (50%) invasive lobular carcinomas (P = .008). However, the alpha v chain detected with the antibody 13C2 was weakly to moderately expressed on nonneoplastic epithelium and at a similar intensity in 13 of 19 IDCs and 15 of 17 invasive lobular carcinomas, suggesting that in IDC the alpha v chain may be associated with a different beta chain (possibly beta 1 or beta 5). No correlation between integrin expression and estrogen/progesterone receptor status was found. These data provide further evidence that in invasive breast carcinomas there is a widespread deregulated expression of integrins and their accessory adhesion molecules with loss of polarization. Changes in the expression and function of cell adhesion molecules, which control growth and differentiation, may have clinical relevance in the behavior of breast cancer.