During the last few years, HLA class I tetramers have been successfully used to demonstrate anti-vaccine CD8 CTL proliferation in cancer patients vaccinated with tumor antigens. Frequencies of CTL as low as 10(-6) among CD8 cells were observed even in patients showing tumor regression. Little is known about the role of tumor-antigen-specific CD4 T cells in the context of these anti-vaccine responses. Therefore, we developed a very sensitive approach using fluorescent class-II-peptide multimers to detect antigen-specific CD4 T cells in vaccinated cancer patients. We produced HLA-DP4 multimers loaded with the MAGE-3(243-258) peptide and used them to stain ex vivo PBL from melanoma patients injected with dendritic cells pulsed with several class I and class II tumor antigenic peptides, including the MAGE-3(243-258) peptide. The multimer(+) CD4 T cells were sorted and amplified in clonal conditions; specificity was assessed by their ability to secrete IFN-gamma upon contact with the MAGE-3 antigen. We detected frequencies of about 1x10(-6) anti-MAGE-3.DP4 cells among CD4 cells. A detailed analysis of one patient showed an anti-MAGE-3.DP4 CD4 T cell amplification of at least 3000-fold upon immunization. TCR analysis of the clones from this patient demonstrated a polyclonal response against the MAGE-3 peptide.