IT has long been assumed that an elastic component other than extracellular collagen fibres is present in muscle fibres to explain their elastic properties, especially during passive stretch1,2 The presence of such an elastic component has been demonstrated in skinned fibres of frog skeletal muscle3,4. Furthermore, it has been observed that myofibrils after the removal of myosin are reversibly extended, indicating their structural continuity5. The chemical entity responsible for the intracellular elasticity of muscle has, however, remained obscure. When rabbit myofibrils were thoroughly extracted with salt solutions such as 0.6 M KI, it was noticed that remaining Z lines still maintained their continuity, although nothing could be seen between the adjacent Z lines under a phase microscope (compare ref. 6). Starting from this observation, we have been able to isolate an elastic protein from myofibrils which is clearly distinguishable from elastin or collagen.