A plasmid DNA encoding an immunogenic peptide derived from the cancer-testis antigen NY-ESO-1 with potential immunostimulating and antitumor activities. Upon adninstration, NY-ESO-1 plasmid DNA cancer vaccine may stimulate the host immune system to mount a cytotoxic T lymphocyte (CTL) response against tumor cells expressing the NY-ESO-1 antigen, resulting in tumor cell lysis. NY-ESO-1 is a tumor associated antigen (TAA) found in normal testes and expressed on the surfaces of various tumor cells, including melanoma, breast, bladder, prostate, lung, ovarian, and hepatocellular tumor cells.