Possible reasons for the heterogeneity of tropomyosin melting curves are considered. This phenomenon is due to the molecular heterogeneity of the preparation, different states of the SH-groups and variations in the stability of different molecular regions. The melting curves of .alpha.-tropomyosin and 2 of its fragments are obtained. The thermodynamic parameters stabilizing their helical structure are determined. A thermodynamical transition at 31.degree. C is shown for .alpha.-tropomyosin leading to the loss of the ability of the molecule to form supra-molecular structures.