Current therapeutic approaches to cancer are designed to target molecules that contribute to malignant behavior but leave normal tissues intact. β₁ integrin is a candidate target well known for mediating cell-extracellular matrix (ECM) interactions that influence diverse cellular functions; its aberrant expression has been implicated in breast cancer progression and resistance to cytotoxic therapy. The addition of β₁ integrin inhibitory agents to breast cancer cells at a single-cell stage in a laminin-rich ECM (three-dimensional lrECM) culture was shown to down-modulate β₁ integrin signaling, resulting in malignant reversion. To investigate β₁ integrin as a therapeutic target, we modified the three-dimensional lrECM protocol to approximate the clinical situation: before treatment, we allowed nonmalignant cells to form organized acinar structures and malignant cells to form tumor-like colonies. We then tested the ability of β₁ integrin inhibitory antibody, AIIB2, to inhibit tumor cell growth in several breast cancer cell lines (T4-2, MDA-MB-231, BT474, SKBR3, and MCF-7) and one nonmalignant cell line (S-1). We show that β₁ integrin inhibition resulted in a significant loss of cancer cells, associated with a decrease in proliferation and increase in apoptosis, and a global change in the composition of residual colonies. In contrast, nonmalignant cells that formed tissue-like structures remained resistant. Moreover, these cancer cell–specific antiproliferative and proapoptotic effects were confirmed in vivo with no discernible toxicity to animals. Our findings indicate that β₁ integrin is a promising therapeutic target, and that the three-dimensional lrECM culture assay can be used to effectively distinguish malignant and normal tissue response to therapy. (Cancer Res 2006; 66(3): 1526-35)