The growth of a potentially antigenic tumor in an immunocompetent host is taken as an indication that the malignant cells 'escaped' the cytotoxic capacity of the immune system. An increase in our knowledge of the means by which antigens are processed and expressed allows a greater understanding of the mechanism that enables tumor cells to avoid host immunity. It provides a rationale for new, more innovative forms of treatment. Antigenic determinants are presented to cytotoxic T lymphocytes (CTLs) in the context of class I determinants, structures specified by genes within the major histocompatibility complex. Potentially antigenic tumors may express lower levels of class I determinants than surrounding non-neoplastic cells. As a consequence, the tumor-associated T cell epitopes formed by the malignant cells may go unrecognized by tumor-specific CTLs. The introduction of genes specifying class I determinants into low class I expressing tumor cells increased class I expression and restored the cells' immunogenic properties. Treatment of low class I expressing cells with interferon-gamma, or the introduction of the interferon-gamma gene into the cells resulted in an increase in the expression of class I determinants as well, and, as a consequence, recognition of the malignant cells by the immune system. Nevertheless, an immunotherapeutic strategy that stimulated a single anti-tumor effector mechanism might be unable to eliminate a heterogeneous tumor cell population. To investigate this point, mice with melanoma were treated with a mixture of interferon-gamma-secreting and IL-2-secreting cellular immunogens. The animals survived significantly longer than mice with melanoma treated with either the IL-2 or interferon-gamma-secreting immunogens alone. The complexity of the problem was illustrated by the fact that although survival was prolonged, tumor growth recurred in each instance.