Quail cells were immortalized for the first time by using retroviruses expressing the 12S adenoviral E1A gene. In these cells, interaction between the 12S E1A product and the quail RB protein was shown, suggesting that the 12S adenoviral E1A product works in avian cells through similar biochemical pathways as in mammalian cells by interacting and inactivating host cellular proteins, including the RB product. These results confirm that the RB product exhibits a universal function among higher vertebrates in controlling cellular growth and tumor progression.