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Interleukin (IL)-12 is a cytokine originally identified from medium conditioned by an Epstein-Barr virus transformed cell line. IL-12 has been shown to increase IFN-γ secretion from NK and T cells, significantly enhance cytolytic activity in both of these cell types, and promote the development of Th1/Tc1 immune responses. These properties make IL-12 an attractive candidate for the development of various clinical protocols ranging from the treatment of viral diseases to tumor immunotherapy. The initial attempts to use IL-12 in the treatment of tumors demonstrated toxicity at potentially therapeutic doses. To circumvent the toxicity associated with IL-12 administration, the authors have developed an adoptive immunotherapy protocol that uses IL-12 for a brief period during ex vivo T cell activation. They show that IL-12 conditioning may be achieved without altering the growth characteristics of the in vitro expanding T cells. T cells generated in the presence of IL-12 show a shift to a Th1/Tc1 dominant phenotype. The resultant cells are more potent killers in vitro and in vivo as assessed by CTL assays and tumor regression. The ability to harness the potent Th1/Tc1 generating potential of IL-12 while avoiding its associated in vivo toxicity has the potential to benefit a large number of clinical trial protocols using adoptive transfer of T cells specific for tumors, viruses, or intracellular pathogens.