At Merck and Co. we have developed a recombinant E1 deficient adenovirus type 5 vaccine vector for HIV-1 and have adopted the PER.C6 cell line as a cell substrate for the manufacture of this vector for Phase I and II clinical trials. The PER.C6 cell line was developed at Crucell by the transfection of human primary embryonic retinoblasts with a transgene of E1 constructed with a minimum of E1 coding sequences to preclude homologous recombination generating replication-competent adenovirus, between E1 sequences in PER.C6 and adenovirus vectors with E1 deletions of the same molecular coordinates. We have developed a Master Cell Bank (MCB) of PER.C6 cells under serum-free conditions of suspension culture from a vial of PER.C6 cells obtained from Crucell. This MCB has been released according to an extensive panel of testing for the detection of adventitious viral agents, including assays for sterility and mycoplasma, in vivo and in vitro assays for the detection of viruses of human, bovine and porcine origin, replication competent adenoviruses, sensitive PERT assays for the detection of RT in supernatants of co-cultivations, electron microscopy and a panel of PCR-based assays for specific human and animal viruses. This MCB has been used for the manufacture of vaccine vector supporting a number of IND submissions for Phase I clinical trials over a three-year period during which the panel of PCR testing applied to the MCB has been judiciously expanded. Advances in QPCR technology, liquid handling systems, and more recently mass spectrometry offer the possibility that very broad panels of primers and probes capable of the detection of all known human viruses can be applied routinely to support the release of biologicals for human clinical trials. The impact of this breadth of testing on the continued reliance of classical in vivo and in vitro assays for adventitious viruses is clearly an emerging issue worthy of serious debate.