Synergy (or antagonism) between two chemical agents is an in vitro empirical phenomenon, in which the observed effect of the combination is more (or less) than what would be predicted from the effects of each agent working alone. Although mathematical synergy is not directly provable in the clinical setting, it does predict a favorable outcome when the two therapeutics are combined in vivo and strongly suggests the presence of in vivo synergy. In contrast, overt antagonism warns of future problems. Sophisticated three-dimensional statistical modeling was used to evaluate the presence of synergistic, additive, or antagonistic efficacy between adenovirus (Ad)-mediated p53 gene therapy (p53 Ad) and paclitaxel (Taxol) in a panel of human tumor cell lines. Cells were either pretreated with paclitaxel 24 h before p53 Ad or treated with both agents simultaneously. Cell proliferation was measured 3 days later. Paclitaxel had synergistic or additive efficacy with p53 gene therapy. In no case was the interaction antagonistic. Cell cycle analysis demonstrated that p53 Ad arrested cells in G0/G1 prior to apoptotic cell death, whereas paclitaxel arrested cells in G2-M prior to apoptotic cell death. When combined, the relative concentration of each agent determined the dominant cellular response. These results are consistent with the previously reported cell cycle effects of p53 or paclitaxel, respectively; however, these data fail to explain the observed drug synergy. We found that low concentrations of paclitaxel (1-14 nM) increased the number of cells transduced by recombinant Ad 3-35% in a dose-dependent manner, which is one possible mechanism for the observed synergy. Of particular note, the concentrations of paclitaxel responsible for increased Ad transduction were lower than the concentrations required for microtubule condensation. The efficacy of combination therapy was also evaluated in vivo. In the p53null SK-OV-3 xenograft model of ovarian cancer, a dosing schedule of p53 Ad that, by itself, had a relatively minimal effect on tumor burden (16%) caused a much greater decrease in tumor burden (55%) when combined with paclitaxel. Greater combined efficacy was also observed in the p53mut DU-145 prostate, p53mut MDA-MB-468 breast, and p53mut MDA-MB-231 breast cancer xenograft models in vivo. In summary, p53 Ad for cancer shows enhanced efficacy when combined with paclitaxel. This combination is recommended for clinical cancer trials.