Effect of increasing duration of culture with IFN-γ on expression of MHC class I and cytotoxicity against K562 cells by bcr-abl–specific clones (P210 and P218) and Mart-1–specific (M81) T cells. The percentage of K562 cells expressing HLA-A,B,C at each time interval in culture with IFN-γ is shown. K562 cells express minimal HLA class I in the basal state and are not lysed by bcr-abl–specific clones (P210 and P218). With increased time in culture with IFN the percentage of K562 cells expressing class I increases as does the mean number of class I molecules on each HLA-A,B,C positive cell as determined by fluorescence intensity. Specific cytotoxicity increased with duration in culture. Cytotoxicity data represents the mean ± SD percent specific lysis in chromium release assays performed in triplicate at an E:T ratio of 10:1. *Significant lysis (P < .006).