EBV episomes are nuclear plasmids that are stably maintained through multiple cell divisions in primate and canine cells (J. L. Yates, N. Warren, and B. Sugden, Nature 313:812-815, 1985). In this report, we describe the construction and characterization of an E1-deleted recombinant adenovirus vector system that delivers an EBV episome to infected cells. This adenovirus-EBV hybrid vector system utilizes Cre-mediated, site-specific recombination to excise an EBV episome from a target recombinant adenovirus genome. We demonstrate that this vector system efficiently delivers the EBV episome and stably transforms a large fraction of infected canine D-17 cells. Using a colony-forming assay, we demonstrate stable transformation of 37% of cells that survive the infection. However, maximal transformation efficiency is achieved at doses of the El-deleted recombinant adenoviruses that are toxic to the infected cells. Consequently, E1-deleted vector toxicity imposes a limitation on our current vector system.