Mutations in troponin, an important muscle protein complex, can result in cardiomyopathy by interfering with the normal muscle activity of the heart. Troponin T (TnT) is the largest subunit of troponin and is involved in binding the troponin complex to the thin filament. Investigation of two mutations associated with cardiomyopathy in TnT, I90M and R173Q, showed different physiological characteristics. The TnT I90M mutation was identified as the causative agent of familial hypertrophic cardiomyopathy (FHC) in a large multi-generational Chinese family and at least two family members with this mutation died of sudden cardiac death. Another mutation in TnT, R173Q, was identified as the underlying cause of dilated cardiomyopathy (DCM). Patients with the TnT R173Q mutation experienced prenatal onset DCM and supraventricular tachycardia at a young age. Functional troponin complexes containing wild-type or mutant TnT's demonstrated similar maximal actomyosin ATPase activity. The inhibitory ability of the troponin complexes containing the I90M mutation was significantly reduced relative to wild-type TnT. Most RCM mutations investigated to date showed a reduced ability to inhibit actomyosin ATPase activity but the RCM mutation, R173Q, did not affect the inhibitory ability of troponin. The mutations showed increased (I90M) and decreased (R173Q) calcium sensitivity of actomyosin ATPase activity consistent with what has been observed for most FHC and DCM mutations. The mutations reduced the rate of degradation of these proteins by calpain relative to wild-type TnT. Overall, these results suggest that although calcium sensitivity may be an indicator of the type of cardiomyopathy no clear trends in maximal or minimal ATPase activity exist that can be used to characterize DCM and FHC mutations.