A critical dose of tumor antigens is required for long-term eradication of tumor. Groups of 4 CBA mice were inoculated with (A) 10⁶, (B) 10⁵, (C) 10⁴, or (D) 10³ A31 lymphoma cells, intravenously on day 0, and were treated on day 10 with either 5 Gy TBI, anti-CD40 mAb (1 mg intravenously), or a combination of the two, as before. Controls received PBS. Survival was recorded daily. Length of protection correlated to tumor burden, with long-term survival completely lost at an inoculating dose of 10³ lymphoma cells. Shown is 1 of 2 similar experiments.