IL-10–producing memory T cells suppress proliferation of autologous PBMCs in an IL-10–dependent manner. CFSE-labeled PBMCs were stimulated with SEB or medium only in the absence or presence of graded amounts of purified autologous IL-10–producing cells, followed by culture for 6.5 days. The ratio of added IL-10–producing cells to PBMCs is indicated. Adding IL-10–producing cells caused inhibition, as indicated in percentage of original proliferation. Where indicated, neutralizing antibodies to IL-10 were added at the beginning of the experiment, which preserved most of the proliferation. Proliferation was assessed by flow cytometry as CFSE^low cells in percentage of total CFSE⁺CD3⁺CD4⁺ T cells. One representative experiment of 3 is shown.