Injection of mice with irradiated A20 cells secreting GM-CSF inhibits the growth of preexisting tumor. Two groups of mice (seven per group) were challenged with 1 × 10⁵ A20 cells on day 1, followed on days 4, 11, and 18 by subcutaneous injection of 1 × 10⁵ irradiated (1,000 cGy) A20 cells that had been transduced with the DISC-HSV GM-CSF vector or the control DISC-HSV β-GAL vector at 30 pfu/cell. The data are reported as mean tumor volume (mm3) ± SE. The difference in tumor growth patterns in the presence of GM-CSF or β-GAL was highly significant (P < .005 by the Wilcoxon test).