Activation of human-purified T cells can be mediated by pairwise combinations of monoclonal antibodies directed against T11.1 and T11.2 epitopes on the CD2 molecule. Monoclonal antibodies (mAbs) reactive with either the alpha and beta chains of the lymphocyte-function-associated antigen-1 (LFA-1) molecule or one of its ligands, intercellular adhesion molecule-1 (ICAM-1), were found to accelerate anti-CD2-induced proliferation. This effect was seen on thymocytes and resting or preactivated T cells (phytohemagglutinin blasts and alloproliferative T cell clones) and could be observed, following the introduction of anti-LFA-1 or -ICAM-1 mAbs, up to 50 hr after the CD2 stimulatory signal. This effect was equally abrogated by 55 kDa anti-interleukin-2 (IL-2) receptor mAb, but neither the expression of IL-2 receptor nor the production of IL-2 was modified. The effects of anti-LFA-1 or anti-ICAM-1 on T cell activation through the CD2 pathway were therefore opposite to those observed in the CD3 pathway, where both mAbs strongly delayed T cell proliferation.