We have identified an adenovirus type 5(Ad5) early gene function located in early region 1 which is required for the production of early cytoplasmic mRNAs corresponding to early regions 2, 3, and 4. Mutant dl312 (lacks the segment between 1.5 and 4.5 map units) grows as well as wild-type virus in 293 cells (Ad5-transformed human embryonic kidney cells), but its growth is severely restricted in HeLa cells. We detect no viral RNAs in the cytoplasm of dl312-infected HeLa cells. Viral RNA sequences are present, however, in dl312-infected HeLa cell nuclei.