e have developed a number of monoclonal antibodies to mouse T ymphocyte cell surface antigens which play critical roles in he process of T cell activation. One group of these reagents s directed to the receptor for interleukin-2 (IL-2) on ctivated murine lymphocytes. One of these antibodies, 7D4, etects an epitope on the IL-2 receptor distal to the ligand inding site, while a second antibody, 3C7, specifically nhibits the binding of radiolabeled IL-2 and is reactive with n epitope near the IL-2 binding site. As 3C7 and 7D4 act ynergistically to inhibit IL-2 driven proliferation, a mixture f the two antibodies has been used to demonstrate that IL-2 lays a major role in all T cell proliferative responses in itro and is very likely a universal growth hormone for T ells. The anti-IL-2 receptor antibodies have also been used o demonstrate the role of non-T accessory cells in the nduction of IL-2 receptor expression on certain T cell subsets nd to demonstrate that IL-2 may play an important role as a ate acting B cell differentiation factor. The mmunosuppressive agent, cyclosporin A, was shown to exert some f its immunological effects by inhibiting the maturation of eceptor expression on activated T lymphocytes. Two other urine T cell differentiation antigens have also been studied ith the aid of newly developed monoclonal antibodies. Thy-1 as shown to play a role in the activation process perhaps by unctioning as a transducing molecule. L3T4, a marker for a ubpopulation of T cells, was also shown to play a role in ransmitting the activation signal. Monoclonal antibodies to ll of these antigens should be attractive candidates for in ivo therapeutic use in attempts to selectively modulate or brogate an ongoing immune response.