The routine transformation of human CD8(pos) T cells by Herpesvirus saimiri has so far not been achieved in the case of pre-expanded antigen-specific CTLs. Here we transformed 73% of polyclonal EBV-specific CD8(pos) T cell cultures using an optimized culture medium supplemented with IL-2, IL-7, IL-12, and TGF-beta(1). Still, antigen-specific cytotoxicity was frequently lost and analysis of the TCR Vbeta-chain repertoire revealed a variable outgrowth of several initially subdominant populations. Limiting dilution cloning of cells in the presence of high titers of HVS did not result in clonal transformation but in the rapid loss of the viral genome in outgrowing clones. In summary, our data suggest that transformation of CD8(pos) T cells out of bulk cultures can be routinely achieved, while viral transformation itself remains an infrequent event on a per cell basis. The practical use of the improved immortalization of antigen-expanded CD8(pos) T cell lines, however, is limited by the arbitrary outgrowth of subdominant populations of unpredictable specificity.