Rejection of A20 wild-type tumors occurs in the absence of tolerogenic APCs and is CD4⁺ dependent. Bone marrow cells (4 × 10⁶) from BALB/c mice (H2^d)or C57BL/6 mice (H2^b) were injected intravenously into lethally irradiated F1 recipients (H2^dxb). (A) Three months after bone marrow reconstitution, mice from H-2^d → H-2^dxb or H-2^b→ H-2^dxb chimeras were injected intravenously with 1 × 10⁶ A20WT tumor cells. Ten mice were included in each group, and they were inspected twice weekly for the development of tumor (P < .01 for the comparison between both chimeras). (B) H-2^b→ H-2^dxb chimeras were treated with anti-CD4- or anti-CD8-depleting antibodies (3 times per week or received no treatment [no depletion]) before challenge with A20WT tumors. Mice received depleting antibodies once a week thereafter until the end of the experiment. Ten mice were included in each group (P < .01 for CD4-depleted vs -nondepleted groups).