The therapeutic effect of TNF gene-transduced mouse fibrosarcoma cells (Meth-A: C5) on pre-inoculated parental cells (Meth-A: M0) was studied. Subcutaneous (s.c.) transplantation of M0 cells into one flank of syngeneic BALB/c mice was followed by s.c. injection of irradiated MO or C5 into the opposite flank 1 week later. The initial M0 tumor (T-MO) completely regressed in C5-vaccinated mice, whereas in M0-vaccinated mice continuous growth of T-M0 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 M0 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 1 and anti-Lyt 2 also demonstrates the involvement of cellular immunity in tumor regression.