Undegraded tyrosine aminotransferase was purified to near homogeneity from rat liver and was confirmed to be a substrate for the beef heart cyclic AMP dependent protein kinase catalytic subunit. Specific antibody was used to quantitate the amount of phosphate incorporated into the enzyme. Phosphate incorporation was maximal at a catalytic subunit to tyrosine aminotransferase molar ratio of 7:1 using 200 microM ATP for 30 to 60 min at 30 degrees C. Phospho-peptide maps of tyrosine aminotransferase phosphorylated in vitro by the catalytic subunit were compared with those of amino-transferase immunoprecipitated from 32P labeled cells treated with and without 8-Br cAMP. Whereas the phospho-peptide maps of tyrosine aminotransferase isolated from cells treated with and without 8-Br cAMP were identical, differences were observed in the peptide map of tyrosine aminotransferase phosphorylated in vitro and in vivo. These results were taken to indicate that the catalytic subunit is not responsible for tyrosine aminotransferase phosphorylation in vivo.