The outcome of clinical corneal transplantation depends on the degree of vascularization and inflammation present in the graft bed at the time of the operation, but the reason for this is unclear. Normal, diseased, and rejected human corneas have been examined with an immunoperoxidase staining procedure, employing monoclonal antibodies to class I and II major histocompatibility complex (MHC) antigens and to other leukocyte markers. In particular, departures from normal in the expression of MHC antigens and in the passenger cell distribution in the diseased or rejected corneas were sought. MHC antigen expression did not alter with inflammation, vascularization, or rejection. However, dendritic-like passenger cells, which were found in low numbers throughout the central stroma of normal cornea as well as in basal epithelium, significantly increased in number in vascularized corneas. We suggest that the breakdown of corneal privilege in vascularized eyes may reflect the increased number of accessory cells in the graft bed.