BACKGROUND Cancer gene therapy with interleukin-12 (IL-12) has generated much interest because of the potent antitumor effects of this cytokine. The purpose of this study was to construct a nonreplicating, noncytopathic recombinant vaccinia virus (irecVV) encoding murine IL-12 (ivKT0327mIL-12) and to assess its biologic activity and antitumor effects. METHODS ivKT0327mIL-12 was constructed by inserting the genes encoding mIL-12 into the VV genome by homologous recombination. Psoralen and long-wave ultraviolet light were used to render the virus incapable of replication or cell lysis. Cytokine production was assessed by infecting tumor cell lines and measuring mIL-12 in the culture supernatants by using a bioassay. C57BL/6 mice were injected subcutaneously in a randomized, blinded fashion with 5 x 10(5) MCA 101 fibrosarcoma cells or 5 x 10(5) PAN 02 pancreatic tumor cells infected in vitro with either ivKT0327mIL-12 or ivKT033, a control irecVV containing no cytokine genes. Emergence of tumor and tumor size were measured. RESULTS Tumor cells infected with ivKT0327mIL-12 produced large amounts of biologically active mIL-12 (up to 793 ng/10(6) cells/24 hr) but continued to proliferate in vitro with no cytopathic effect. Mice injected with MCA 101 infected with ivKT0327mIL-12 had significantly delayed tumor emergence (p < 0.03) and decreased tumor size (p < 0.003) compared with mice injected with ivKT033-infected MCA 101. Tumor growth was also significantly smaller in a similar experiment with PAN 02 (p < 0.01). CONCLUSIONS Tumor cells infected with irecVV encoding mIL-12 express high levels of biologically active mIL-12 in vitro and exhibit delayed tumor establishment, significant tumor growth inhibition, or both, in vivo.