Adenoviruses (Ad) have served as outstanding agents in contributing to an understanding of transformation and regulation of gene expression in cultured cells (reviewed in Tooze 1981; Doerfler 1983, 1984; Ginsberg 1984). Three decades after their original isolation (Rowe et al. 1953) 41 serotypes of Ad which cause infections in over a dozen species have been identified. Initial classification was based on immunological criteria and on turmorigenicity in immunocompetent rodents, which ranges from highly oncogenic (e.g., Ad 12) to generally nononcogenic (e.g., Ad2 and Ad5). More recently DNA homology has also been included as a criterion. Because they provided the first example of a human virus capable of inducing tumors in experimental animals and because Ad is capable of transformation in vitro, there was obvious, immediate interest in them. Subsequent revelation that the Ad heavily employs much of the cellular machinery during its life cycle made the adenovirus family an excellent probe for dissecting macromolecular biosynthesis and expanded this interest. Adaptation of Ad as a vector for various applications was thus a natural consequence of the considerable wealth of information accrued about this virus. Establishing cell lines by transformation with Ad recombinants that contain heterologous genes was one obvious application. Another was its development as a vector for producing high level expression of heterologous genes, stimulated by the extremely efficient lytic expression of this virus.