RhD protein, but not the control recall antigen PPD, is presented to T cells by CLL B cells. T cells were purified from PBMCs, and CD5⁺ CLL B cells were positively selected from the remaining cells. The ability of the T cell–depleted PBMCs and the CD5⁺ CLL B cell fraction to stimulate T cell proliferation in response to RhD protein was compared. CLL B cells are more effective than PBMCs in presenting RhD protein to T cells from patients CLL+AIHA2 (A), CLL+AIHA5 (B), CLL+AIHA6 (C), and CLL+AIHA10 (D). In contrast, CLL B cells fail to stimulate proliferative responses against PPD by T cells from patients CLL+AIHA1 (E) and CLL+AIHA7 (F). Significant differences between cultures stimulated using unfractionated PBMCs and CD5⁺ CLL B cells as APCs are indicated (Student t test: *P < .05; **P < .01).