The involvement of the alpha 6 beta a integrin, a laminin receptor, in breast carcinoma progression needs to be addressed rigorously. We report that a human breast carcinoma cell line, MDA-MB-435, known to be highly invasive and metastatic, expresses three potential integrin laminin receptors: alpha 2 beta 1, alpha 3 beta 1, and alpha 6 beta 1, but uses only alpha 6 beta 1 to mediate adhesion and migration on laminin matrices. To investigate the contribution of alpha 6 beta 1 to the aggressive behavior of these cells, we developed a dominant-negative strategy for knocking out alpha 6 beta 1 function that involved expression of a cytoplasmic domain deletion mutant of the beta 4 integrin subunit by cDNA transfection. Stable transfectants of MDA-MB-435 cells that expressed this mutant beta 4 subunit were inhibited dramatically in their ability to adhere and migrate on laminin matrices, and their capacity to invade Matrigel was reduced significantly. These findings support the hypothesis that alpha 6 beta 1 is important for breast cancer progression. Moreover, this approach is a powerful method that should be useful in assessing the role of alpha 6 beta 1 in other cells.