Both Tc1 and Tc2 populations clonally delete precursor CTL in vitro by a veto-type mechanism. A mixed lymphocyte reaction was established in 24-well plates using a 10:1 mixture of responder spleen cells from C57Bl/6(H-2^b) and C57Bl/6 transgenic mice (2C mice; CD8⁺ T cells are transgenic at the TCR locus for L^d allospecificity) and stimulator spleen cells from DBA/2 mice (H-2^d). The yield of transgenic CD8⁺ T cells in the MLR (tg responder MLR) was calculated by determination of cell counts and transgene percentage (transgenic TCR was identified by flow cytometry after staining with FITC-labeled 1B2 antibody); in this system, the transgenic CD8 population expands appoximately 10-fold between days 2 and 3 of the MLR. To evaluate the ability of Tc1- and Tc2-type cells to clonally delete this transgenic population by a veto mechanism, CD8⁺ T cells from B6D2F1 donor mice were stimulated in vitro with irradiated spleen cells from B6C3F1 mice under Tc1 or Tc2 conditions, harvested on day 7 of culture, and added to the transgenic MLR at the indicated numbers per well (tg + Tc1 and tg + Tc2) on day 2 of the MLR. The yield of transgenic CD8 cells was then determined on day 3 of the MLR.