Systemic administration of a recombinant adenovirus encoding the human interferon-beta gene (H5.110CMVhIFN-beta) results in transduction of hepatocytes and detectable circulating levels of IFN-beta protein. In preclinical studies in mice, we noticed a distinctly nonlinear dose response, with low levels of virus (1-3 x 10(10) viral particles) yielding barely detectable levels of IFN-beta but with a higher viral dose (1 x 10(11) particles) resulting in disproportionately high IFN-beta levels. Further studies showed that transgene expression levels from low viral doses could be dramatically enhanced by coadministering an unrelated recombinant adenovirus (H5.110CMVlacZ), suggesting that there was a viral dose threshold effect for efficient viral transduction and/or IFN-beta expression. This enhancement of reporter expression by a nonreporter adenovirus, effective upon coadministration, was further enhanced by preadministration of H5.110CMVlacZ (up to 8 h), but was ineffective if the helper virus was administered as little as 5 min after the H5.110CMVhIFN-beta reporter virus. Our data suggest that the reticuloendothelial system plays a role in this threshold effect, such that low doses of virus are efficiently taken up by the RES/Kupffer cells without leading to appreciable transgene expression, whereas high doses saturate these cells and are able to productively transduce hepatocytes. A better understanding of this phenomenon could have an impact on gene therapy clinical trial safety and efficacy.