Monocytic CD11b⁺Gr-1⁺ cells are sufficient for the generation of CTL-suppressive cells in vitro. Splenocytes from either naive AKR or tumor size–matched BW-Sp3 progressors, with or without anti–Gr-1 treatment, were restimulated in vitro with irradiated BW-Sp3(B7-1) cells for 5 days. To test the function of adherent cells, a comparison was made between untouched cultures (Total) or cultures where the nonadherent cells were removed from the adherent population after 2 days, followed by another 3-day restimulation of the nonadherent cells (Nad). The cytotoxic activity in all cultures was tested using ¹¹¹In-labeled BW-Sp3(B7-1) cells as targets at a 100:1 effector-target ratio. Results show the average ¹¹¹In-release ± SD of triplicates. Similar results were obtained from 3 independent experiments.