The reversible thermally induced unfolding of various forms of tropomyosin, a two-chain a-helical coiled coil, has been studied by high-sensitivity differential scanning calorimetry (DSC) . Included in the study are the reduced and oxidized (disulfide cross-linked) forms of am-and PP-tropomyosin, and the forms of aa-tropomyosin in which all sulfhydryl groups have been blocked by carboxymethylation or carboxyamidomethylation. Oxidation or blocking of the sulfhydryl groups of tropomyosin strongly affect the thermotropic behavior of the protein in unpredictable ways. The empirical results presented here are in qualitative agreement with those from an earlier DSC study of the oxidized and carboxymethylated forms of aa-tropomyosin [ S. A. Potekhin and P. L. Privalov (1982) Journal of Molecular Biology, Vol. 159, pp. 519-5351, but we find that a different decomposition into subtransitions is possible. Comparison of the aa and pP species indicates, in agreement with extant CD studies, that the noncross-linked Pp species is somewhat less stable than its aa counterpart, but that cross-linking enhances the stability of the pp doubly cross-linked species by a greater amount and does not lead to the small low-temperature transition ("pretransition") seen in the singly cross-linked aa species.