Interactions between cells and extracellular matrices are mediated in part by a family of heterodimeric molecules known as integrins. We have investigated, using immunohistology, the distribution of six integrin alpha sub-units in normal breast tissue and 26 breast carcinomas. Alpha-1 integrin (collagen/laminin receptor sub-unit) was detected in myoepithelium, but not in luminal epithelium nor in most (20/26) carcinomas. Its expression on fibroblasts was enhanced in demoeplastic stroma. Both benign and malignant epithelium showed uniform positive staining for alpha-2 (collagen receptor sub-unit) and for alpha-3 (collagen/fibronectin/laminin receptor sub-unit). All epithelium was negative for alpha-4 (sub-unit of a fibronectin receptor). Epithelial staining for alpha-5 (fibronectin receptor sub-unit) was weak in all samples. Alpha-6 (sub-unit of two integrin laminin receptors) showed conspicuous changes in all invasive carcinomas. In normal tissues, there was weak staining of epithelial cytoplasm with alpha-6 antibody and moderate cell membrane staining. Strongest staining was present in a basement membrane distribution. In carcinomas, loss of cytoplasmic and cell membrane staining was variable, but basal membrane staining was diminished or absent in all tumours. Loss of basal membrane staining for alpha-6 integrin corresponded closely to loss of immunoreactivity for its ligand laminin in invasive breast cancer.