The half-life of the adenovirus 5 early region 1A (E1A) proteins was examined in productively infected and transformed cells. In HeLa cells infected with adenovirus 5, the half-life of the E1A proteins was approximately 30 min; in the transformed 293 cells, the constitutively expressed E1A proteins had a half-life of approximately 120 min. In HeLa cells, the E1A proteins produced by an adenovirus mutant that expresses only the 13S mRNA had a half-life of about 35 min; E1A proteins produced by a mutant that express only the 12S mRNA had a half-life of about 80 min. This difference in the stability of these two classes of E1A proteins helps explain why the steady-state level of the 12S class is usually equal to or greater than that of the 13S class, despite the fact that the concentration of the 13S mRNA is about four times greater than the concentration of the 12S mRNA.