Bovine cardiac troponin is similar to rabbit skeletal troponin with respect to secondary structure, amino acid composition and molecular weight of the subunits, but differs slightly with respect to biological activity and surface charges of the subunits. Previous circular-dichroic studies of the subunits and recombination of subunits have indicated significant Ca2+-induced delocalized conformational changes. Present studies of the native troponin complex are not in accord with such changes. Furthermore the formation of the troponin-tropomyosin complex in vitro results in no delocalized conformational changes, nor does it sensitize troponin to Ca2+-induced changes. It is suggested that the troponin complex cannot be dissociated into subunits without significant and irreversible conformational perturbation.