Previously, we reported that macrophage (H-2DbKb)-mediated cytotoxicity of allografts [e.g., BALB/c skin (H-2DdKd)] in the rejection site and that we isolated two cDNAs encoding novel receptors on mouse macrophages or monocytes for H-2Dd and H-2Kd (mouse MHCs) and named them MMR1 and 2. Recently, we identified human homologs of mouse MMR1 and 2 by a search of sequence homology (BLAST) and also isolated two cDNAs encoding human MMR1 and 2 from human PBMCs cDNA library by RT-PCR method. In this meeting, we present the identification and characterization of human MMR1. The predicted amino acid sequence of human MMR1 cDNA (approximately 1.5 kb) encoded a putative single transmembrane protein, and the homology showed 64% identity to the entire mouse MMR1 protein and 96%, 96%, and 98% identity to its respective intracellular, transmembrane, and MHC binding regions. RT-PCR analyses demonstrated that human MMR1 expressed on monocytes in PBMCs. To examine the binding toward HLAs (human MHCs), HEK293T cells expressing human MMR1 were incubated with the beads conjugating 80 kinds of HLA class I or fluorescein-labeled soluble HLA molecules and then were analyzed by flow cytometer. Flow cytometric analyses showed that the mean fluorescence intensity of human MMR1 protein toward HLA-B44 was significantly high and that HEK293T cells expressing human MMR 1 bound specifically to HLA-B44 with a dissociation constant of 3.0 x 10-9 M. These results suggest that human MMR 1 on monocytes or macrophages might be novel receptors for HLA-B44.