We have previously reported the identification of a glycoprotein dimer, now termed CD100, at the lymphocyte cell surface using a mAb termed BB18. In the present study we isolated a second mAb, termed BD16, recognizing another epitope of this 150 kDa structure. We show that interaction with individual mAb triggers differential functional effects on peripheral blood lymphocytes (PBL) and purified T cells. In particular and of the utmost interest, BD16 mAb strongly inhibits CD3 induced PBL proliferation while it dramatically increases the CD2 induced proliferation. In contrast, after T cell purification, BD16 mAb increases both CD2 and CD3 induced proliferation. It has to be noted that perturbation of the same 150 kDa surface molecule with BB18 mAb exerts no effect on CD2 and CD3 induced proliferations, while inducing a strong lymphocyte proliferation in the presence of submitogenic concentrations of phorbol myristate acetate.