The integrin superfamily represents a major class of receptors mediating cell-substrate adhesion. Our recent study of the tissue distribution of the alpha 2 beta 1 integrin, a cell-surface collagen receptor, revealed that high levels of receptor expression were associated with orderly, regulated epithelial cell proliferation. Those observations prompted the present investigation of alpha 2 beta 1 and other integrins in adenocarcinoma of the breast. The alpha 2 beta 1 integrin was highly expressed on the epithelium of the ducts and ductules of normal breast tissue. Normal or nearly normal levels of the receptor were expressed in fibroadenomas. In contrast, markedly decreased or undetectable alpha 2 beta 1 expression was typical of poorly differentiated adenocarcinomas. Well-differentiated lesions exhibited intermediate levels of expression. Similar, but less extensive, decreases in expression were observed for the alpha 5 beta 1 (fibronectin receptor) and alpha v beta 3 (vitronectin receptor). Significant expression of the beta 1 subunit on even poorly differentiated tumors suggests that the expression of other undefined members of the beta 1 family is not reduced to the same low level as alpha 2 beta 1 and alpha 5 beta 1. Expression of the alpha 2 beta 1 integrin was highly correlated with estrogen-receptor expression. Decreased expression of alpha 2 beta 1 and other integrin adhesive protein receptors probably contributes to the altered adhesive properties of tumor cells characteristic of the malignant phenotype.