The MDM-2_53-61 and p53_264-272 peptides specifically cold target inhibit CTL recognition of HPLC fractions 48 and 53 respectively. ⁵¹Cr-labeled T2 cells were pre-loaded with peptides from either PCI-13 HPLC fraction 48 or 54, washed and then added to microwells containing CTL-DC#1 at an effector-to-target cell ratio of 2:1. To each of these wells were then added either media, unlabeled T2 cells or T2 cells preloaded with either the p53_264-272 or MDM-2_53-61 peptides at a cold:hot T2 ratio of 10:1. After 4 h incubation at 37°C, supernatants were harvested, radioactive counts (cpm) determined and the mean percentage (± SD) of cold target inhibition calculated from triplicate determinations. The data presented are from one assay of two performed which yielded comparable results.