CD4⁺ CD25⁺ cells prevent expansion of hapten-specific CD8⁺ T cells. Ii^°/° mice were either left untreated or were transferred intravenously on day –8 with either naive total CD4⁺ T cells (10 × 10⁶), CD4⁺CD25^– T cells (9 × 10⁶), or CD4⁺CD25⁺ T cells (1 × 10⁶), fed with DNFB on day –7 and skin sensitized on day 0 with DNFB. Purified CD8⁺ T cells (2 × 10⁵) from spleen and lymph nodes, harvested on day 5 after sensitization, were restimulated in vitro for 3 days with syngeneic mitomycin C–treated spleen cells (5 × 10⁵) either untreated or pulsed with DNBS. T-cell proliferation was determined by [³H]thymidine uptake during the last 8 hours. Results are expressed as Δcpm values (ie, cpm from hapten-derivatized spleen cells – cpm from untreated spleen cells) ± SD of triplicate wells.