Adenoviral vectors are attractive for the delivery of transgenes into mammalian cells because of their efficient transduction, high titer, and stability. The major concerns with using E1-deleted adenoviral vectors in gene therapy are the pathogenic potential of the virus backbone and the leaky viral protein synthesis that leads to host immune responses and a short duration of transgene expression. Helper-dependent (HD) adenoviral vectors that are devoid of all viral protein-coding sequences have significantly increased the safety and reduced the immunogenicity of these vectors. Currently available HD vectors depend on an E1-deleted adenovirus as a helper to provide viral proteins in trans. As a consequence, contamination with helper virus cannot be avoided in the HD vector preparation though it can be decreased to 0.01% using a Cre/loxP mechanism. Since the presence of E1-deleted helper virus may have substantial unwanted effects, we have developed a new Cre-expressing cell line based on an E1- and E2a-complementing cell. This new cell line can efficiently cleave the packaging region in the helper virus genome. We have also developed an E1 and E2a double-deleted helper virus. By using the CreE cell with the helper virus deleted in both the E1 and the E2a genes it may be possible to further improve the safety of the vectors.