Calpain 1, a ubiquitous calcium-dependent intracellular protease, was recently found in a tight association with myofibrils in skeletal muscle tis-sue [Delgado EF, Geesink GH, Marchello JA, Goll DE & Koohmaraie M (2001) J Anim Sci79, 2097–2107). Our immunofluorescence and immuno-electron microscopy investigations restrain the protease location at the per-iphery of the Z-band and at the midpoint of the I-band.