The proliferation of T cells against Kell peptides is dependent on HLA-class II molecules. Cultures of PBMCs from (A-B) alloimmunized donor 3 or (C) control donor 5 were stimulated with Kell peptide 1M(_179-193) or 3M(_181-195), and class II–restricted responses were blocked by addition of Ab against all HLA-DP, DQ, and DR molecules, or specific only for DR. Isotype control Ab was added in selected experiments. P < .05 was taken as significant inhibition (t test on log-transformed data). The results are representative of 4 alloimmunized and 4 control donors.