Current generation adenovirus (Ad) vectors are deleted for the E1 region of genes and require propagation in E1 expressing 293 cells. Expression of genes delivered by Ad vectors into immunocompetent hosts is generally transient since the current vectors are not completely replication defective. Viral proteins expressed by Ad vectors, in part, induce a rapid, T cell-mediated loss of the transduced cells. Introduction of temperature-sensitive point mutations into new Ad vectors may be of limited usefulness in prolonging transduced gene expression in vivo. Isolation of new Ad vectors deleted for genes required for normal Ad growth may further prevent Ad protein expression. These new vectors will need to be grown in 293 cells capable of coexpressing other Ad genes. Unfortunately, many of the Ad genes are toxic when coexpressed in 293 cells. We describe the isolation of E1 expressing 293 cells which also express both the Ad polymerase and preterminal proteins, both of which are essential to normal Ad growth. The isolation of new Ad vectors deleted for the E1, polymerase and preterminal proteins are predicted to have many advantageous properties, including the prolongation of transduced foreign gene expression in vivo.