Disclosed are compositions and methods for repairing and/or regenerating cardiac tissue by administering adult bone marrow-derived stem cells to an individual. These cells can be administered as a liquid injectible or as a preparation of cells in a matrix which is or becomes solid or semi-solid. The cells can be genetically modified to enhance myocardial differentiation and integration. Also disclosed is a method for replacing cells ex vivo in a heart valve for implantation.