Using a chemically homogeneous radiolabeled peptide of high specific activity ( 1 2 5 l-QLSPYPFDL, 3.5 x 10 1 8 cpm per mole) we show that at a peptide concentration (5 pM) causing half-maximal lysis of target cells by a cytolytic T lymphocyte (CTL) clone that recognizes the peptide in association with L d , a class I MHC protein, only 3 peptide molecules on average are bound by L d per target cell. From the distribution of L d on the target cells, we suggest that a single peptide-MHC complex per target cell can trigger activation of the T cell cytolytic response.