Interferons upregulate the expression of HLA class I antigens on cancer cells. Nevertheless, little is known about the panel of HLA class I antigen-associated peptides presented by recombinant alpha-interferon (r(alpha)-IFN)-treated cells. For this reason, peptides were eluted from five cancer cell lines (four melanoma and one non-small cell lung cancer) following treatment with r(alpha)-IFN. High-performance liquid chromatography (HPLC) profiles of the peptide fractions were compared with those obtained from untreated cells. No significant differences in peptide characteristics (detectable on the basis of retention times) were observed, but significant differences in terms of peptide quantities were observed. Mass spectrometry performed on HPLC peaks allowed not only the detection of three different peptides (two derived from the MAGE family of genes and one from the mart-1) both in untreated and in treated cells, but also gave an indication of the number of peptides within one HPLC peak. This data demonstrates that r(alpha)-IFN-treated cells express a similar peptide pattern as untreated cells, with significant quantitative differences. Interestingly, this finding also explains the higher susceptibility to lysis (mediated by specific cytolytic lymphocytes, which recognize cancer cells in an HLA-restricted fashion) of r(alpha)-IFN-treated cells.