Aromatic L-amino acid decarboxylase (AADC) was purified from bovine adrenal medulla and properties of this enzyme were compared with those of AADC from human pheochromocytoma. The molecular weights of the subunits were identical between human and bovine enzymes and estimated to be 50,000 by SDS-polyacrylamide gel electrophoresis. An isoelectric point of the human enzyme was 5.7, while the bovine enzyme showed several distinct bands at the region of pH 4.9-5.3 in the absence of urea. Multiplicity of the isoelectric point of bovine AADC disappeared in the presence of urea. These results showed that there were some differences between the properties of human and bovine AADC in spite of the high homology (88%) in their primary structures.