Alpha Actinin is an actin binding protein present in both muscle and non-muscle cells. It has a molecular weight of 100,000 daltons and forms dimers in solution. In normal skeletal muscle alpha actinin is associated with the z-discs that define muscle sarcomeres. In smooth muscle alpha actinin has been detected in dense bodies and plaques characteristic of the tissue. Immunofluorescent labeling of a large variety of cells using antibody to alpha actinin reveals extensive association of the proteins with actin containing stress fibers and in particular with their membrane bound termini.