In a cancer gene therapy model recombinant adenoviruses challenge with wild-type tumor but not from challenges with expressing the herpes simplex virus thymidine kinase an unrelated syngeneic tumor cell line. Since cytotoxic T (HSVtk) gene were injected into tumors in situ, either alone lymphocyte responses in this tumor model were weak, we or in combination with adenoviruses (Avs) engineered to analyzed cytokine secretion from spleen cells of treated express IL-2, IL-6 or the costimulatory molecule B7-1.animals. The best correlate of antitumor immunity in this HSVtk phosphorylates the prodrug ganciclovir, thus conmodel was enhanced secretion of GM-CSF, while verting it into an antimetabolite which kills not only HSVtk secretion of IL-2, IL-6 and IFN␥ was also frequently expressing cells, but also by the 'bystander effect', neighincreased but not as consistently. The enhanced IFN␥ boring untransduced tumor cells. The tumors regressed in secretion associated with unchanged IL-4 secretion sug-80% of mice upon AvTK/ganciclovir treatment; combigests that AvTK treatment results in a predominantly Th1-nations with AvIL-2, AvIL-6, or AvB7-1 did not improve mediated antitumor immune response. these results. Cured mice were protected from further