The thermal stability of the matrix protein (M protein) of Newcastle disease virus (NDV) has been investigated using high-sensitivity differential scanning calorimetry (DSC) at pH 7.4. The thermal folding/unfolding of M protein at this pH value is a reversible process involving a highly cooperative transition between folded and unfolded monomers with a transition temperature (Tm) of 63 °C, an unfolding enthalpy, ΔH(Tm), of 340 kcal mol(-1), and the difference in heat capacity between the native and denatured states of the protein, ΔCp, of 5.1 kcal K(-1) mol(-1). The heat capacity of the native state of the protein is in good agreement with the values calculated using a structure-based parameterization, whereas the calculated values for the hypothetical fully-unfolded state of the protein is higher than those determined experimentally. This difference between the heat capacity of denatured M protein and the heat capacity expected for an unstructured polypeptide of the same sequence, together with the data derived from the heat-induced changes in the steady-state fluorescence of the protein, indicates that the polypeptide chain maintains a significant amount of residual structure after thermal denaturation.