A panel of sera raised against NK-1.1 and NK-2.1 alloantigens was tested for reactivity against a panel of cloned antigen-dependent CTL lines. By using indirect immunofluorescence and flow cytofluorimetry, weak, but clear and consistent, reactivity was found on all CTL. Concordant with the genetics of NK alloantigens, C57BL/6-derived clones were reactive with anti-NK-1.1 and anti-NK-2.1 sera, whereas CBA-derived clones were reactive with anti-NK-2.1 sera but not with anti-NK-1.1 sera. Cloned CTL lines were also able to partially and specifically absorb the antibodies from NK alloantiserum that reacted with splenic NK cells. These results indicate that cloned CTL lines express at least some of the NK alloantigen determinants present on splenic NK cells and have important implications regarding the relationship of CTL and NK cells.