Identification of the antigenic peptide. ⁵¹Cr-labeled HLA-Cw*1601 LB678 EBV-B cells were incubated with peptides for 15 min before addition of CTL clone 82/21 at an effector to target ratio of 5 to 1. Chromium release was measured after 4 h. The concentrations indicated in the figure are the concentrations during the 4-h incubation. The arrow indicates the concentration at which half-maximal lysis was observed.