A series of BALB/c-derived T hybridoma cells, capable of producing interleukin 2 (IL 2) in response to poly(Glu60,Ala30,Tyr10) (GAT) presented by syngeneic B lymphoma cells in the context of Ad restriction determinants, was used as a model system to evaluate the roles of LFA-1 and L3T4 accessory molecules in antigen-specific T cell activation. Examination of the antigen requirement for optimal IL 2 responses revealed marked differences in the apparent avidity of these cells for GAT/Ad complexes. A relationship was observed between this parameter and the susceptibility of T hybridoma cells to inhibition by monoclonal antibodies directed at 5 distinct epitopes of LFA-1, and at Apd allodeterminants. In contrast, L3T4a-specific monoclonal antibodies were found to block in a similar fashion the antigen-specific IL2 responses of T hybridoma cells, regardless of the apparent avidity of their antigen receptors. It was also shown that both L3T4' and L3T4-T hybridoma cells were capable of recognizing GAT plus Ad with high avidity. Thus, the quality of T cell antigen recognition appears to critically influence the involvement of LFA-1, and only to a marginal extent that of L3T4, in antigen-specific T cell activation. The implications of these findings are discussed in the context of recent data indicating that L3T4 may not only be an Ia-binding protein.