This is the first report on peptidic inhibitors of heme oxygenase. Such peptides were originally developed from the immunomodulatory peptide 2702.75-84 which corresponds to amino acid residues 75 to 84 of the alpha1-helix of HLA-B2702 (2702.75-84) and has been shown to be immunosuppressive in vitro and in vivo. In vitro, 2702.75-84 inhibited cytotoxic T- and natural killer cell- mediated target cell lysis, and in vivo peptide therapy resulted in prolongation of heart and skin allograft survival in mice. The peptide was also shown to bind to heat shock protein 70. However, D-enantiomers of 2702.75-84 and derivatives thereof, while still being immunosuppressive, did not bind to heat shock protein 70. This study was designed to identify proteins binding to peptide D2702.75-84(E --> V) (rvnlrialry) consisting of D-amino acids. Compared with 2702.75-84 (RENLRIALRY), glutamic acid residue 76 (E) was replaced with valine (V). Affinity chromatography using immobilized D2702.75-84(E --> V) and mouse and human cell extracts, resulted in the isolation of heme oxygenase-1 (HO-1). Peptide D2702.75-84 inhibited HO activity in vitro in a dose dependent manner. Similar to what has been observed with other inhibitors of HO, administration of peptide into mice resulted in an up-regulation of HO-1 mRNA and protein, as well as enzyme activity in liver, spleen and kidney. Other peptides derived from 2702.75-84 with similar immunomodulatory activity displayed similar effects. In contrast, inactive derivatives of 2702.75-84 had no effect on HO activity. Therefore, the immunosuppressive effects of the described immunomodulatory peptides are similar to those of cobalt-protoporphyrin, a known up-regulator of HO-1. Our results suggest that HO-1 modulation may be a novel mechanism of immunomodulation.