Cancer immunotherapy often aims at the reactivation and expansion of tumor-specific CTL. In an attempt to correlate in situ and/or systemic tumor-specific T cell expansion with tumor regression, we investigated the effects of adenovirus-mediated IL-12 or IFN-gamma gene transfer into established P815 murine tumors. While IFN-gamma was no more potent than the vector alone, IL-12 gene transfer promoted tumor eradication. Despite this antitumor effect, no significant cytolytic activity was detectable using classical cytotoxicity assays from in vitro restimulated splenocytes. Since intratumor gene delivery may induce a localized expansion of CTL, the presence of P815-specific CD8+ T cells in situ was assessed. Using the Immunoscope approach, we found a dramatic increase in clonotypic T cells at the tumor site following IL-12, but not IFN-gamma gene delivery. Antitumor CD8+ T cell frequencies were then re-evaluated using this molecular detection technique, which revealed a comparable expansion of specific T cells in the peripheral organs, most strikingly in the blood. These data show that local IL-12 gene transfer, in contrast to IFN-gamma, mediates a potent antitumor effect that correlates to clonal tumor-specific T cell expansions in situ and in the periphery.