Monoclonal antibody (mAb) blocking assays using L3T4- and lymphocyte function-associated antigen-1 (LFA-1)-specific reagents were used as an approach to investigate the involvement of these accessory molecules in various T cell activation pathways. As previously reported, rat mAb to L3T4a and LFA-1A functional epitopes efficiently blocked antigen-driven T helper cell proliferation. In contrast, antigen- and Ia-independent T cell triggering induced by appropriate mAb to the Thy-1 or the T cell receptor molecules were found to be inhibitable by L3T4a- but not LFA-1A-specific mAb, although the extent of inhibition varied, depending on the cell type and the activating signal examined. These results provide further evidence that the inhibiting effects of L3T4-specific mAb on T cell responses may be due, in addition to an impairment of L3T4-class II major histocompatibility complex molecular interaction, to a down regulatory signal possibly transmitted by the L3T4 molecule itself.