A peptide derived from the α1 domain of the human HLA class I heavy chain (amino acids 75–84; B2702. 75–84) has been shown to inhibit human cytotoxic T and NK cell activity in a non-allele-restricted manner. In vivo, this peptide prolonged skin allograft survival in a murine model. Here we demonstrate prolongation of heart allograft survival in mice and extend the characterization of the immunomodulatory activity of B2702. 75–84. Similar to what has been observed with retrovirus-derived peptides, the inhibitory capability of this peptide was increased when bound to a carrier protein. An increased immunomodulatory activity was also observed with the dimeric peptide B2702. 84–75-75–84 or the multimeric B2702. 75–84. MAP. This peptide not only inhibited cytotoxic T and NK cells but also anti-CD3-induced T cell proliferation as well as a mixed lymphocyte reaction (MLR). Flow cytometric analysis of T cells harvested from anti-CD3-stimulated spleen cell culture in the presence of B2702. 84–75-75–84 showed decreased expression of activation markers (CD25, ICAM-1, Pgp-1, CD69) compared with untreated control cultures. The superior activity of B2702. 84–75-75–84 could also be demonstrated in vivo. Administration of B2702. 84–75-75–84 prolonged the survival of B6 (H2 b) hearts in CBA (H2 k) recipients to 15±2.7 (P= 0.0002 vs. control) days compared with 11.4±2.6 (P= 0.01) days in B2702. 75–84 treated animals and 7.5±1.1 days in untreated controls. Administration of control peptides had no significant effect on allograft survival. In combination with a subtherapeutic dose of cyclosporine, B2702. 75–84 induced longterm graft survival in 60% of recipients.