A number of cytokines and costimulatory molecules involved in immune activation have recently been identified including IL-12, a heterodimeric cytokine that supports the development of cell-mediated immunity, and B7-1, a costimulatory molecule involved in the activation of T lymphocytes. We explored the use of these immunomodulants as molecularly defined adjuvants in the function of recombinant anticancer vaccines using a murine model adenocarcinoma, CT26, transduced with a model Ag, beta-galactosidase (beta-gal). Although IL-12 given alone to mice bearing tumors established for 3 days did not have consistent antitumor activity, a profound therapeutic effect was observed when IL-12 administration was combined with a recombinant vaccinia virus (rVV) encoding beta-gal called VJS6. On the basis of the reported synergistic effects of IL-12 and the costimulatory molecule B7-1 (CD80) in vitro, we immunized mice with a double recombinant vaccinia encoding both the model tumor Ag and the costimulatory molecule B7-1, designated B7-1 beta-gal rVV. The adjuvant administration of IL-12 after immunization with this virus significantly enhanced survival in tumor-bearing animals. T cell subset depletions demonstrated that the in vivo activity of IL-12 was largely independent of CD4+ T lymphocytes, whereas the in vivo activity of a B7-1 rVV required both CD4+ and CD8+ T cells to elicit maximal therapeutic effect. To our knowledge, this is the first description of B7-1 and IL-12 cooperation in vivo and represents a novel strategy to enhance the efficacy of recombinant anticancer vaccines.