1. 1. Tyrosine aminotransferase has been purified from chicken liver to homogeneity by a 5-step procedure. The resultant enzyme preparation has a specific activity (256 units activity/mg protein) comparable to results published for the enzyme purified from rat liver and represented an overall recovery of 35–40%. 2. 2. In terms of structure (native and subunit molecular weights, immunological reactivity, and kinetic parameters) (apparent Michaelis constants for l-tyrosine and 2-oxoglutarate, oxoacid specificity, pH optimum) the purified enzyme from chicken liver exhibits remarkable similarities to tyrosine amino-transferase from rat liver.