An specific transaminase for valine, leucine and isoleucine with α-ketoglutarate was purified from mitochondria of hog heart and its properties were compared with those of the similar enzyme from the supernatant. The purified mitochondrial enzyme was shown to be a single protein by ultracentrifugal, electrophoretic and immunological procedures. Its sedimentation constant, optimum pH, SH requirement and substrate specificity were similar to those of the supernatant enzyme. However, the Km values of the mitochondrial enzyme for branched chain amino acids were one tenth of those of the supernatant enzyme, while that for α-ketoglutarate was four times larger than that of the supernatant enzyme. The Km values of the two enzymes for pyridoxal phosphate were also significantly different. A difference between the two enzymes was also shown in their stability. Thus the supernatant enzyme was more stable than the mitochondrial one on heat treatment or aging. The supernatant enzyme was eluted slightly ahead of the mitochondrial enzyme from a Sephadex G-100 column. The two enzymes have different electrophoretic mobilities on acrylamide gel plate. These results indicate that the branched chain amino acid transaminase of hog heart mitochondria is specific for these amino acids like the supernatant enzyme, but the protein natures of the two are different.