The NY-ESO-1 gene is expressed by a range of human tumors and encodes HLA-A2-restricted melanoma peptides recognized by CD8+ CTLs. Here we report that the NY-ESO-1 gene also encodes two overlapping, but non-cross-reactive, HLA-DRB1*0401-presented peptides that are recognized by CD4+ T cells. The NY-ESO-1(119-143) peptide was able to induce specific CD4+ T cells in vitro from both an HLA-DRB1*0401+ normal donor and an HLA-DRB1*0401+ patient with melanoma. Bulk and cloned CD4+ T cells produced IFN-gamma specifically in response to, and also lysed, T2.DR4 cells pulsed with peptide NY-ESO-1(119-143) and the autologous tumor cell line, but not a DRB1*0401+ melanoma cell line that does not express NY-ESO-1. Interestingly, the NY-ESO119-143 peptide contains two overlapping putative "core" epitopes recognized by non-cross-reactive anti-NY-ESO-1(119-143) CD4+ T-cell clones. Taken together, these data support the use of this novel DR4-restricted tumor peptide, NY-ESO-1(119-143), or its two "sub-epitopes" in immunotherapeutic trials designed to generate or enhance specific CD4+ T-cell responses against tumors expressing NY-ESO-1 in vivo.