Both DST and heart allograft are required to tolerize recipient splenic Th cells. Spleen and LN cells were prepared from DST-treated and untreated heart graft recipient rats killed 5 days after the transplantation. Proliferation against irradiated spleen from LEW.1W (RT1.u) or third-party Buffalo (RT1.b) and Lewis (RT1.l) (data not shown) was assessed every day as described in Fig 1. Representative data of five independent experiments are shown.