Pleural exudative lymphocytes (PLEL) from a 60-year-old female patient showed high cytotoxicity against the autologous mammary tumor line, HMC-2, and NK-susceptible K562 cells, although PLEL demonstrated only weak cytotoxic potentials against several allogeneic tumor lines. We successfully obtained seven cytotoxic T cell clones from PLEL bulk populations, and assessed the possibility that these lymphocytes are simply natural killer (NK)-like cells or have the dual cytotoxic activity of cytotoxic T lymphocytes (CTL) and NK-like cells. These clones, designated as TcHMC-2, showed strong cytotoxicity against both HMC-2 and K562 cells. In contrast, allogeneic human peripheral blood-derived NK cells could not kill HMC-2 targets. Furthermore, a blocking study of TcHMC-2 cytotoxicity using monoclonal antibodies against CD3, CD8 and human MHC class I products showed that all of these antigen molecules were involved in the cytotoxicity of TcHMC-2 clone against autologous HMC-2 cells, indicating MHC class I recognitive cytotoxicity. These data indicate that the TcHMC-2 clone may have dual cytotoxicity with CTL- and NK-like activity against autologous HMC-2 mammary tumor and K562 cells, respectively.