The phenotype of a differentiated cell results from the expression of a unique set of genes in that cell. The differentiation of F9 teratocarcinoma cells in response to retinoic acid and cyclic AMP is an excellent example of this process, as the appearance of several gene products during the course of the differentiation process has been documented. In principle, the activation of gene expression could be due to the appearance of positive-acting factors, the loss of negative-acting factors, or a combination of both. Since F9 cells have been shown to express a cellular E1A analog whereas differentiated F9 cells do not, and it is known that the viral E1A gene exerts a negative effect on transcription of both viral and cellular genes, we determined whether the cellular genes activated during F9 cell differentiation are subject to E1A negative control. We found that infection of differentiated F9 cells with wild-type adenovirus resulted in a decline in the levels of collagen type IV mRNA and plasminogen activator mRNA, both of which are induced by differentiation. At least for the collagen gene, this phenomenon appears to involve a transcriptional repression.