To distinguish the individual roles of the 13S, 12S, and 9S adenovirus E1A gene products, we isolated the corresponding cDNA clones and recombined them into both plasmids and viruses. Only the expected E1A mRNA products were made from the corresponding 12S and 13S viruses. The 9S mRNA was detected when the 9S virus was coinfected with the 13S virus but not when either virus was infected alone. The 13S virus formed plaques equally well in 293 cells, HeLa cells, and A549 cells, a human lung oat cell carcinoma line. Plaque titers of the 12S virus were much reduced in HeLa and A549 cells compared with 293 cells, although the 12S virus is multiplicity-dependent leaky in both HeLa and A549 cells. A549 cells were significantly more permissive than HeLa cells for growth of the 12S virus. In A549 cells even at low multiplicities of infection the final yield of 12S virus eventually approached the maximum yield from 293 cells. Expression from the adenovirus early region 2 and early region 3 promoters in HeLa cells was activated in the presence of a 13S cDNA E1A region but not in the presence of a 12S E1A cDNA region. Although defective for lytic growth in HeLa cells, the 12S virus immortalized BRK cells at very high efficiency, whereas infection of these cells with 13S virus, as with wild-type E1A virus, resulted mainly in cell death. The 13S product does have an immortalization function, however, revealed in the absence of adenovirus lytic functions when a plasmid containing the E1A 13S cDNA region was transfected into BRK cells. The 9S virus failed to immortalize infected BRK cells or to interfere with focus formation when coinfected with the 12S virus.