Administration of recombinant interleukin 12 (IL-12) induces tumor regression that is associated with T-cell infiltration in the OV-HM ovarian carcinoma and CSA1M fibrosarcoma models. After confirming the blocking of regression by injection of anti-IFN-γ monoclonal antibody (mAb), we investigated the mechanisms underlying the requirement of IFN-γ in T-cell migration and tumor regression. T-cell migration was inhibited by injection of anti-IFN-γ mAb to OV-HM tumor-bearing mice prior to IL-12 treatment. We examined, using the lymphoid cell migration assay, whether IFN-γ is required for enhancing the migratory capacity of T cells or the T cell-accepting potential of tumor masses during IL-12 treatment. Spleen cells from IL-12-treated or untreated OV-HM-bearing mice were stained in vitro with a fluorescein chemical and transferred i.v. into OV-HM-bearing mice that were not treated with IL-12. Migration of donor cells was quantitated by counting the number of fluorescent cells on cryostat sections of tumor masses from recipient mice. Compared to spleen cells from OV-HM-bearing mice that were not treated with IL-12, enhanced migration was observed for cells from IL-12-treated OV-HM-bearing mice. Anti-IFN-γ pretreatment of donor mice before IL-12 treatment did not reduce the migratory capacity of T cells, whereas migration was markedly inhibited in recipient mice injected with anti-IFN-γ. Anti-IFN-γ pretreatment decreased vascular cell adhesion molecule-1 (VCAM-1)-/intercellular adhesion molecule-1 (ICAM-1)-positive blood vessels at tumor sites. Consistent with this, migration was also inhibited by treatment of recipient mice with either anti-VACM-1 or anti-ICAM-1 mAb. In contrast to the OV-HM model, T-cell migration was not affected in the CSA1M model following preinjection of anti-IFN-γ mAb. In this model, VCAM-1-/ICAM-1-positive blood vessels existed even after anti-IFN-γ treatment, although tumor regression was completely inhibited. These results indicate that IFN-γ plays two distinct roles in expressing the antitumor efficacy of IL-12: one is to support the T-cell acceptability of tumor masses, and the other is to mediate the antitumor effects of migrated T cells.