Extracellular matrix (ECM) in solid tumors affects the effectiveness of therapeutics through blocking of intratumoral diffusion and/or physical masking of target receptors on malignant cells. In immunohistochemical studies of tumor sections from breast cancer patients and xenografts, we observed colocalization of ECM proteins and Her2/neu, a tumor-associated antigen that is the target for the widely used monoclonal antibody trastuzumab (Herceptin). We tested whether intratumoral expression of the peptide hormone relaxin (Rlx) would result in ECM degradation and the improvement of trastuzumab therapy. As viral gene delivery into epithelial tumors with extensive tumor ECM is inefficient, we used a hematopoietic stem cell (HSC)-based approach to deliver the Rlx gene to the tumor. In mouse models with syngeneic breast cancer tumors, HSC-mediated intratumoral Rlx expression resulted in a decrease of ECM proteins and enabled control of tumor growth. Moreover, in a model with Her2/neu-positive BT474-M1 tumors and more treatment-refractory tumors derived from HCC1954 cells, we observed a significant delay of tumor growth when trastuzumab therapy was combined with Rlx expression. Our results have implications for antibody therapy of cancer as well as for other anticancer treatment approaches that are based on T-cells or encapsulated chemotherapy drugs.