The cellular immortalization activity of cloned genes can be identified either in a colony-forming assa of transfected primary rat embryo fibroblasts or in a co-operation assay together with ras. However the demonstration of immortalization activities carried by cellular genes has not been reported. Here we establish that SV40 early genes integrated in genomic DNAs can be stably transferred into rat embryo fibroblasts and selected via their immortalization activity. Attempts to extend this assay to the identification of dominant genes putatively involved in the immortality of several other immortal post-crisis or tumor cells have been unsuccessful suggesting that the immortal phenotype can be brought about through different pathways.