A cell-cell contact is required for suppression of CD8+ T lymphocyte response. Gr-1+ cells were enriched through panning with a specific mAb from the spleens of mice that had received TS/A tumor inoculation. Following culture in medium without cytokines for 7 days, adherent cells were harvested and added at a final concentration of 0.18 × 106 cells (3%) to a MLC consisting of BALB/c splenocytes and γ-irradiated C57BL/6n splenocytes. The Gr-1+-derived cells were either admixed in the same well (cell contact) or separated by a semipermeable membrane (no cell contact). Control cultures did not receive the third part population of suppressor cells (shown in the leftmost panel). After 5 days, cytolytic activity in the cultures was assayed against either a syngeneic (H-2d) target, LSTRA, or an allogeneic (H-2b) target, MBL-2. This experiment was repeated twice with similar results.