MHC molecules present peptides in their binding groove to T-cell receptors inducing proliferation or cytotoxicity of alloreactive T cells. A previously generated human monoclonal antibody (mAb) UL-5 A1, recognizing a conformational epitope formed by HLA DR1/DRB1*0101 molecules and HLA-A2 derived peptides, demonstrates T-cell-like recognition of the peptide/MHC complex (PMC). To study the genes of the antigen binding region, the nucleotide sequences of the rearranged genes in the variable regions of UL-5 A1 were determined and the V-gene usage (VH3, V lambda 2) was identified by comparison with published germlines. The genes encoding heavy (Fd) and light (L) chains of UL-5 A1 were linked and expressed in a bacterial system. Specificity of the recombinant Fab-5 A1 was determined with HLA-typed LCLs by flow cytometric analysis. As demonstrated in competitive inhibition assays, UL-5 A1 and Fab-5 A1 recognize the same PMC epitope on HLA-A2+, -DR1/DRB1*0101+ typed LCLs. Additionally, mAb UL-5 A1 and Fab-5 A1 both recognize HLA-A2-, -DR1/DRB1*0101+ LCLs exogenously loaded with HLA-A2 peptides (105-117, 103-117). UL-5 A1-like antibodies against peptide/MHC complexes could prove valuable tools for research on T-cell recognition and MHC function.