A synthetic peptide vaccine consisting of a HLA-DR15-restricted human Wilms' Tumor protein-1 (WT1) peptide comprised of amino acids 124 through 138, a HLA class II-restricted WT1 peptide, with potential immunomodulating and antitumor activities. Vaccination with WT1 124-138 peptide may stimulate a CD4-positive helper T-lymphocyte-mediated immune response against WT1 expressing cells. Activated helper T-cells stimulate dendritic cells, and activate the proliferation of other T-lymphoctes and B-lymphocytes. This causes tumor cell lysis and inhibition of cancer cell proliferation in WT1-overexpressing tumor cells. WT1, a zinc finger DNA-binding protein, is overexpressed in most types of leukemia and in a variety of solid cancers.