At least four separate enzymes are found to catalyze the transamination between phenylalanine and alpha-ketoglutarate in E. coli K 12, one of them being the aspartate aminotransferase. The Km of the latter enzyme for alpha-ketoglutarate is 0.3 or 0.035 mM according to the acceptor aminoacid being phenylalanine or aspartate respectively. The double specificity of aspartate aminotransferase in E. coli is however clearly shown by thermal inactivation studies using various effectors or different temperatures, and by the finding of an active transamination between aspartate and phenylpyruvate in the absence of ketoglutarate. This reaction shows the usual ping-pong type of mechanism, which implies that both substances are substrates for the same protein. Contrary to the phenylalanine-alpha-ketoglutarate reaction, which is probably of little importance in vivo when catalyzed by this enzyme, the direct ketoglutarate-free transamination between aspartate and the aromatic alpha-ketoacid is likely to represent a physiological function in regulating, at least partially, the balance between biosynthetic pathways for aromatic aminoacids and aspartate, for instance by maintaining similar ratios between the aminoacid and its ketoacid partner in both cases. For the sake of clarity it is proposed that the name "transaminase A", first devised by Rudman and Meister, be used for aspartate aminotransferase only, knowing that the specificity of this peculiar enzyme behaves as an accessory agent in the transamination of the aromatic compounds.