A 9-residued peptide vaccine consisting of amino acids 157 through 165 of the cancer/testis (CT) antigen ESO-1. Modified at position 165 (cysteine to valine) to improve immunogenicity, ESO-1:157-165(165v) peptide administered as a vaccine may stimulate the host immune system to mount a cytotoxic T lymphocyte (CTL) response against tumor cells positive for ESO-1, resulting in decreased tumor growth. ESO-1 is expressed in a variety of cancers, including melanoma, breast, bladder, prostate, and hepatocellular cancers. (NCI04) [ ]