A messenger RNA fraction has been isolated from the liver of cortisol treated rats which when added to an in vitro protein synthesizing sytem gives rise to tyrosine-α-ketoglutarate transaminase activity. Development of enzyme activity is dependent on active protein synthesis and is inhibited by puromycin and erythromycin. It is concluded that the messenger RNA fraction contains the information for the formation of tyrosine transaminase and that the enzyme is formed in vitro by de novo synthesis.