CD4⁺ and CD8⁺ T cells contribute to the inhibition of preexisting leukemia following CD40L/IL-2 costimulation. Four groups of mice (seven per group) were challenged with 10⁵ A20 cells on day 1. On days 4 and 14, the animals were treated with irradiated A20 cells mixed with fibroblasts expressing either the CD40L or IL-2 gene or neo (control). Mice were depleted of T lymphocytes by IP injection of the Gk1.5 (anti-CD4) or 2.43 (anti-CD8) MoAbs 1 day before tumor challenge and again on alternate days for 1 week, and then weekly for 4 weeks. The data are reported as mean mm3 ± SE.