Antigens encoded by MAGE-A3 and recognized by T cells are interesting targets for tumor immunotherapy because they are strictly tumor specific and shared by many tumors of various histological types. A number of MAGE-A3 antigenic peptides presented by HLA class I molecules have been used in clinical trials, and regressions of melanoma metastasis have been observed. We report here the identification of a MAGE-A3 epitope, TQHFVQENYLEY, presented to CD4⁺ T lymphocytes by HLA-DP4 molecules, which are expressed in ∼76% of Caucasians. This new epitope may be useful both for therapeutic vaccination and for the evaluation of the immune response in cancer patients. Interestingly, the CD4⁺ T cells lysed HLA-DP4 tumor cells expressing MAGE-A3,indicating that this epitope, in contrast to other class-II MAGE-A3 epitopes, is presented at the surface of tumor cells. The study of this disparity in the presentation of two epitopes from the same protein may lead to a better understanding of the endogenous class II presentation pathway.