AbstractMonoclonal anti‐Qat‐4 and anti‐Qat‐5 antibodies, which define antigens expressed on peripheral T cell subsets, have been used to study the phenotypes of alloreactive and H‐2‐restricted cytotoxic effector cells and their precursors. Depletion of Qat‐4+or Qat‐5+cells from the T cell pool prior to their sensitization in bulk cultures prevented the development of alloreactive and H‐2‐restricted cytotoxic activities in the selected populations. No reconstitution of cytolytic activities to normal levels was obtained when mixtures of Qat‐4+and Oat‐5+cells were sensitized in bulk cultures to H‐2 or non‐H‐2 antigens. Sensitization of limiting numbers of Qat‐4−or Qat‐5−lymphocytes under optimal conditions for help (interleukin 2), with the appropriate antigens (H‐2, H‐Y) did not result in the generation of cytotoxic T cells, indicating that the majority of all cytotoxic T lymphocyte (CTL) precursors are Qat‐4+, Qat‐5+.When CTL effector populations were treated with the antisera and complement (C) at their maximum CTL activity, it was found that H‐2‐restricted CTL were totally eliminated by anti‐Qat‐4 and considerably reduced by anti‐Qat‐5 antisera and C. In contrast, alloreactive CTL effector cells were insensitive to anti‐Qat‐4 and to anti‐Qat‐5 plus c. Although alloreactive CTL effector populations regained some Qat‐4 antigens during furtherin vitroculture, it was shown that H‐2‐restricted CTL were at all times more sensitive to anti‐Qat‐4 than were alloreactive CTL. The findings suggest that during maturation of alloreactive and H‐2‐restricted CTL from their precursors, both alloantigens undergo differential quantitative variations in their expression t