The proliferative response during MLC is mediated by CD4⁺ cells. Spleen cells were prepared from DST-treated and untreated heart graft recipients sacrificed 5 days after the transplantation. Total, CD4-depleted, or CD8-depleted (Dynal) cells (2 × 10⁵) were plated in triplicate with an equal number of Lew.1W (RT1.u) irradiated spleen cells in 96-well round-bottom plates and cultured for 5 days. Thymidine incorporation was assessed during the last 8 hours. Representative data of three independent experiments are shown.