The therapeutic effect of TNF gene-transduced mouse fibrosarcoma cells (Meth-A C5) on pre-inoculated parental cells (Meth-A. MO) was studied. Subcutaneous (s.c.) transplantation of MO cells into one flank of syngeneic BALB/c mice was followed by S.C. injection of irradiated MO or C5 into the opposite flank I week later. The initial MO tumor (T-MO) completely regressed in C5-vaccinated mice, whereas in MOvaccinated mice continuous growth of T-MO was observed. When a similar experiment was carried out in SCID mice, no regression of T-MO was observed, suggesting that the tumor regression in BALB/c mice was not due to direct anti-tumor activity of TNF secreted from C5, but to systemic immunity. Regression of the rechallenged MO tumor was observed in mice which had shown T-MO regression by C5 vaccination, but rechallenged Colon 26 cells (syngeneic to BALB/c mice) continued to grow, indicating a specific immunity to Meth-A cells. The systemic immunity evoked in C5-vaccinated mice was directly demonstrated by enhanced killer activities of LAK and CTL with a proliferation of T-cell population in their splenocytes. Abrogation of the therapeutic effect of C5 vaccination with anti-Thy I and anti-Lyt 2 also demonstrates the involvement of cellular immunity in tumor regression. d 1995 Wiley-Liss, hic.