Antigen-specific helper T cells recognize a complex of peptide antigen and class II major histocompatibility complex (MHC) gene products. Whether T cells recognize MHC class II alloantigen by a similar mechanism or the native conformation of MHC molecules themselves has yet to be determined. The demonstration that peptide antigens bind directly and specifically to class II molecules has allowed us to examine the influence of foreign peptide binding on T-cell recognition of allogeneic MHC molecules. We report here that an immunodominant, HLA-DR1-restricted peptide of influenza virus hemagglutinin (HA residues 306-320) is able to modulate the recognition of alloantigen by human DR1-specific T-cell clones. For some T-cell clones, but not all, the HA peptide inhibited allorecognition in a dose-dependent manner. However, in one instance, the proliferative response to alloantigen was enhanced in the presence of HA peptide. These results suggest that the specificities of T-cell responses to allogeneic MHC molecules are heterogeneous, which may be influenced by different peptides occupying the class II MHC combining site and by the diversity of antigen-specific receptors of T lymphocytes recognizing the same MHC/peptide complex.