Using calpastatin antibody we have identified a 145 kDa major band along with two relatively minor bands at 120 kDa and 110 kDa calpastatin molecules in bovine pulmonary artery smooth muscle mitochondria. To the best of our knowledge this is first report regarding the identification of calpastatin in mitochondria. We also demonstrated the presence of micro-calpain in the mitochondria by immunoblot and casein zymogram studies. Immunoblot studies identified two major bands corresponding to the 80 kDa large and the 28 kDa small subunit of mu-calpain. Additionally 76 kDa, 40 kDa and 18 kDa immunoreactive bands have also been detected. Purification and N-terminal amino acid sequence analysis of the identified proteins confirmed their identity as mu-calpain and calpastatins. Immunoprecipitation study revealed molecular association between mu-calpain and calpastatin in the mitochondria indicating that calpastatin could play an important role in preventing uncontrolled activity of mu-calpain which otherwise may facilitate pulmonary hypertension, smooth muscle proliferation and apoptosis.