Human melanoma is a highly immunogenic tumor capable of inducing a specific immune response. A number of melanoma-associated antigens have been characterized during the past several years and can be classified into two groups: differentiation antigens-present also in normal melanocytes-and tumor-specific antigens, which, with the exception of testis, are present only in tumor cells. In a previous publication [Kirkin A. F., Petersen T. R., Olsen A. C., Li L., thor Straten P., Zeuthen J. (1995) Cancer Immunol Immunother 41:71] we have described the production of clones of cytotoxic T lymphocytes (CTL) against the highly immunogenic human melanoma cell line FM3. Using these clones we have defined four previously unknown melanoma-associated antigens, which could be subdivided into differentiation and progression antigens. In the experiments reported in this paper, we have further compared CTL clones from different groups and shown that the sensitivity of melanoma cells to CTL that recognize differentiation or progression antigens is differentially modulated during tumor progression as well as by the lymphokines interferon gamma (IFN gamma) and interleukin-10 (IL-10). The interaction of CTL clones recognizing progression antigens was strongly increased after treatment of melanoma cells with IFN gamma, while the recognition by CTL clones specific for differentiation antigens either was unchanged or significantly decreased. IL-10 treatment of melanoma cells induced up-regulation with respect to recognition by CTL clones specific for differentiation antigens without affecting the recognition of melanoma cells by CTL clones specific for progression antigens. Using cellular systems at different stages of tumor progression, we demonstrated that the progressed state of melanoma cells is associated with increased sensitivity to recognition by CTL clones detecting progression antigens, and with decreased sensitivity to CTL clones recognizing differentiation antigens. Mimicking tumor progression, treatment with IFN-gamma induced apparent down-regulation of differentiation antigens. A hypothesis is suggested in which IFN-gamma plays different roles in the immune response against poorly immunogenic and highly immunogenic melanoma cells, increasing the progression of poorly immunogenic tumor cells or promoting a strong immune response and regression of highly immunogenic melanoma cells.