Abstract 1. 1. Sulphydryl groups of mitochondrial aspartate aminotransferase from horse heart were titrated with 5,5′-dithiobis (2-nitrobenzoic acid). 2. 2. From analysis of peptic peptides, 378 amino acid residues (94.3% of the total) in the protein were identified. 3. 3. The results of amino acid sequence analysis are compared with those of cytosolic and mitochondrial aspartate aminotransferases from other sources.