H-2-lacking murine embryonal carcinoma (EC) cells have been proposed as universal targets for natural killer (NK) effectors from different species because their killing appeared to be uncomplicated by potential T cell effector mechanisms (Stern, P. L. et al., Int. J. Cancer 1981. 27:679). While some previous studies had shown that murine cytotoxic T cells were unable to lyse EC cells, rat T killers are shown here to be active against these targets and to be distinguishable from NK cells. Percoll density fractionation of rat peripheral blood lymphocytes enriches in parallel for NK-mediated lysis of both EC or YAC target cells. These NK cells unlike T cells, do not mediate lectin-dependent and cell-mediated cytotoxicity (LDCC) of NK-insensitive target cells. This procedure is thought to reveal the total cytolytic potential of stimulated T cell populations, regardless of specificity. In contrast to previous results with mice, we found that allogeneically primed rat cytotoxic T cells can kill murine EC cells in LDCC and, further, that rat cytotoxic T cells, generated by stimulation with mouse spleen cells in vitro, can lyse murine EC cells directly. This demonstration of T cell lysis of EC cells suggests that either there is a novel mechanism of lysis operating without requirement for major histocompatibility complex (MHC) structures, or EC cells express some hitherto unidentified MHC-like structures on their cell surface.