When chicken breast muscle myofibrils were treated with a solution containing 0.1 mM CaCl2 and 30 micrograms of leupeptin/ml, nebulin filaments were fragmented into 200-, 180-, 40-, 33-, and 23-kDa subfragments. All the subfragments except the 180-kDa one were released from the myofibrils. The fragmentation of nebulin filaments seems to be induced by the binding of large amounts of calcium ions. Similar changes took place in nebulin filaments in post-mortem skeletal muscle. It has been proposed that nebulin co-exists with thin (actin) filaments and participates in stabilizing their organization [Wang, K. & Wright, J. (1988) J. Cell Biol. 107, 2199-2212]. Thus, the above result suggests that Ca-induced fragmentation of nebulin filaments destabilizes the organization of thin filaments and is a key factor in meat tenderization during post-rigor aging.