Cardiac myosin was isolated from chickens of the following ages: 2-5 days and 14-16 days post-fertilization, newborn and adult. Polyacrylamide gel electrophoresis in the presence of sodium dodecyl sulfate or 8M urea (pH 8.6) showed no differences in heavy or light chains of myosin. There was no difference in the K ion-EDTA activated ATPase activity.