We have detected a cellular factor in F9 teratocarcinoma cells that recognizes an adenovirus E1A inducible promoter. This factor, termed E2F, was previously identified in HeLa cells and was found at increased levels as a function of the E1A gene product. Upon differentiation of F9 cells with retinoic acid and cAMP, the factor declines to near undetectable levels, consistent with the control of this factor by E1A and the presence of a cellular E1A-like activity in F9 cells but not in differentiated F9 cells. Finally, if the E1A gene is introduced into differentiated cells by an adenovirus infection, there is a large increase in the level of the factor. We suggest that the control of E2F during F9 differentiation is indeed due to an E1A-like activity.