During the last decade, the aberrant expression of normal testicular proteins in neoplastically transformed cells became common knowledge. Cancer-testis antigens (CTAs) represent a novel family of immunogenic proteins. The genes MAGE, BAGE, GAGE, LAGE and NY-ESO-1 code for antigens that are recognised on various neoplastically transformed cells by autologous, cytolytic CD8 ( + ) T lymphocytes. The MAGE genes were initially analysed from melanomas and turned out to have an almost exclusively neoplasm specific expression pattern. In normal adult tissues, most 23 human MAGE genes are expressed only in the testis, with expression patterns suggesting that this gene family is involved in germ cell development. The SSX (synovial sarcoma on X chromosome) gene family, located on the X chromosome, encode a family of highly homologous nuclear proteins. A number of observations confirmed that all five SSX genes were expressed in normal testis. The newly detected CTA, NY-ESO-1, is regarded as one of the most immunogenic antigens ever isolated, inducing spontaneous host immune responses in 50% of patients with NY-ESO-1-expressing neoplasms. The identification of neoplasm-associated markers recognised by cellular or humoral effectors of the immune system has opened new perspectives for antigen directed, individualised antineoplastic immunotherapy. In preparation for this new era of targeted immunotherapy, a number of neoplasm-associated antigen families have been identified as targets for CD8+, cytolytic T lymphocytes in vitro and in vivo : (1) CTAs expressed in various neoplasms and in normal testis, restricted to male germ cells; (2) melanocyte differentiation antigens; (3) point mutations of normal genes; (4) antigens overexpressed in neoplastic tissues; and (5) viral antigens. Immunotherapeutic protocols directed against the CTAs have already been initiated to analyse the induction of antigen-specific cellular and humoral immune responses in vivo.