A method of producing recombinant viruses which require helper viruses for packaging is provided. The method uses a recombinant helper virus which has been designed to contain at least one rare-cutting restriction site (e.g., for I-SceI), a viral vector which is helper-dependent for packaging, and a recombinant host cell which is capable of expressing a rare-cutting restriction enzyme (e.g., I-SceI). The method involves transfecting or infecting the host cell with the helper virus and viral vector and incubating the cell under conditions which permit encapsidation of the viral vector in an adenovirus capside. Thereafter, the I-SceI expressed by the host cell digests the helper virus, permitting ready separation of the digested fragments of the helper virus from the packaged recombinant virus.