MHC class I molecules expressed on cell surfaces are composed of H chain, beta2-microglobulin and any of a vast array of peptides. The role of peptide in the recognition of HLA class I by serum HLA Abs is unknown. In this study, the solid-phase assay of a series (n = 11) of HLA-A2-reactive, pregnancy-induced, human mAbs on a panel (n = 12) of recombinant monomeric HLA-A2 molecules, each containing a single peptide, revealed peptide selectivity of the mAbs. The flow cytometry membrane staining intensities on the HLA-A2-transduced cell line K562, caused by these mAbs, correlated with the number of monomer species detected by the mAbs. Flow cytometry staining on HLA-A2-bearing cell lines of a variety of lineages was indicative of tissue selectivity of these HLA-A2 mAbs. This tissue selectivity suggests that the deleterious effect on allografts is confined to alloantibodies recognizing only HLA class I loaded with peptides that are derived from tissue-specific and household proteins. Since Abs that are only reactive with HLA loaded with irrelevant peptides are expected to be harmless toward allografts, the practice of HLA Ab determination on lymphocyte-derived HLA deserves reconsideration.