To produce a helper virus-free stock of helper-dependent adenoviral vectors (HDAdVs), we have developed a new helper system in which adenoviral genes for propagation of HDAdVs are delivered into producer cells by a herpes simplex virus-1 (HSV) amplicon-adenovirus hybrid. The hybrid amplicon was constructed to carry the E1 gene (HA-E1) or the entire adenoviral genome except E1 (HA-Ad). El expression from the HSV amplicon successfully complemented propagation of an El-deleted adenoviral vector in a human glioma cell line. HDAdVs were propagated in 293 cells infected with HA-Ad. In addition, HDAdVs were rescued and propagated in a glioma cell line superinfected with both HA-E1 and HA-Ad amplicons, although relatively low titers of HSV amplicon resulted in low propagation efficiency of HDAdVs. Since the HSV amplicon can be easily and completely inactivated by chloroform extraction and/or heat treatment from the HDAdV stock, this helper system might be an alternative method to produce helper virus-free HDAdVs. (C) 2003 Elsevier Inc. All rights reserved.