Results obtained after digestion of mitochondrial aspartate aminotransferase from pig heart with pepsin and with the protease from S. aureus are described. Peptic digestion produced a very complex mixture of peptides, which were purified and analyzed; structural information contained in these peptides covered nearly the entire molecule. Moreover, the lengths of some individual peptides and the peculiar self-overlapping found with families of peptides from adjacent regions were especially useful and interesting. Not all the possible peptides originating after digestion with S. aureus protease were isolated and examined. However, the high specificity of this protease and its usefulness for sequence studies were confirmed. In particular, the S. aureus peptides obtained were important for establishing the amidation state of glutamic acid/glutamine residues.