The activation requirements for antigen-dependent proliferation of CD4+ T cells are well documented,while the events leading to the inactivation phase are poorly understood. Here, we tested the hypothesis that the lymphocyte-activation gene 3 (LAG-3), a second major histocompatibility complex (MHC) class I1 ligand, plays a regulatory role in CD4+ T lymphocyte activation. CD4+ class 11-restricted T cell clones were stimulated by their relevant antigen (hemagglutinin peptide or diphteria toxoid) and antigen-presenting cells with or without anti-LAG-3 monoclonal antibody (mAb). Kinetic studies were performed to monitor different activation parameters, including the measurement of thymidine incorporation, expression of activation antigens and cytokine secretion. Results showed that the time course from the initial time points up to the peak time point was not modified in the presence of anti-LAG-3 mAb. However, addition of these antibodies, either as whole IgG or as Fab fragments, led to increased thymidine incorporation values for late time points and, hence, to a shift in the decreasing proliferation curve. We also showed that expression of activation antigens, such as CD25, was higher in the presence of anti-LAG-3 mAb, and that cytokine concentrations, i.e. of interferon-y or interleukin4,were higher in the corresponding culture supernatants. In addition, we tested whether the effects of anti-LAG-3 mAb were limited to antigen-dependent, MHC class 11-restricted responses. The proliferative responses of CD4+ T cell clones following stimulation with either interleukin-2, mitogens, a combination of anti-CD2 mAb, immobilized anti-CD3 or anti-T cell receptor mAb were not altered by anti-LAG-3 mAb. The allogeneic proliferative response of a CD8+ Tcell clone was also not affected. Overall, the present analysis reveals a modulating effect of anti-LAG-3 mAb, mediated specifically on antigendependent, MHC class 11-restricted responses of CD4+ T cell lines. These results support the view that L A G 3 M H C class I1 interaction down-regulates antigendependent stimulation of CD4+ T lymphocytes.