Both myosin and actin in the frozen stored myofibrils retained their high salt-solubility, which decreased slowly during the frozen storage period. The presence of aggregated inactivated myosin in the salt-soluble fraction was proven by precipitation at 40 % saturation of ammonium sulfate in the presence of Mg-ATP, leaving active monomeric myosin in the soluble fraction. Almost no actin denaturation was observed with heated myofibrils.