Abstract: We characterized a total of 175 HLA-DR-associated phosphopeptides using sequential affinity isolation, biochemical enrichment, mass spectrometric sequencing and comparative analysis. Many were derived from source proteins which may have roles in cancer development, growth and metastasis. Most were expressed exclusively by either melanomas or transformed B cells, suggesting the potential to define cell type-specific phosphatome “fingerprints”. We generated HLA-DR?1*0101-restricted CD4+ T cells specific for a phospho-MART-1 peptide identified in two melanoma cell lines. These T cells showed specificity for phosphopeptide-pulsed antigen presenting cells as well as for intact melanoma cells. MHC II-restricted phosphopeptides recognizable by human CD4+ T cells are potential targets for cancer immunotherapy.