AbstractThe comparative study of proteins which differ in primary structure by point mutations permits one to use thermodynamic experiments to obtain information about the role of specific amino acids in determining protein structure and stability. We have now determined the thermodynamic changes induced in six mutants of T4 lysozyme and have compared the results with the wildtype enzyme. Our work is in collaboration with B. Matthews and his colleagues, who have determined the crystal structure of T4 lysozyme and have obtained difference Fourier maps for four of the mutants. The ultimate aim is to correlate changes in protein stability with changes in the detailed structure of the protein. This paper discusses the thermodynamic results obtained from the mutants studied. All the mutants have a lower Tm than the wild‐type enzyme and changes in the enthalpy of denaturation are sometimes extraordinarily large. Changes in ΔH of denaturation are usually accompanied by compensating changes in ΔS. The general question of protein stability and the manner in which it varies with temperature and mutations is discussed.