The well-characterized immunogenetic constitution of the rat has made it a valuable resource for the study of corneal transplantation. However, little is known about the expression of histocompatibility antigens in the rat cornea or about the populations of passenger leukocytes rat corneal grafts may contain. Thus the relevance of rat corneal transplantation to the human situation is unclear. We examined the expression of antigens of the histocompatibility complex (MHC) of the rat (RT1) and of antigens identifying rat leukocyte populations in corneas of selected rat strains using a sensitive immunoperoxidase technique. Class I MHC antigens were found in fairly uniform association with most cells of the rat corneal epithelium, endothelium, and stroma. In contrast, class II MHC antigens were not associated with most corneal parenchymal cells, but were clearly present on multiple isolated dendritic cells throughout the corneal stroma, especially near the limbus, and on very rare isolated cells in the limbal epithelium; these antigens were not detectable on the corneal endothelium. Cells expressing an antigen common to all rat leukocytes were present in a distribution similar to that of class II MHC antigen-bearing cells. There was no consistent expression of antigens associated with T lymphocytes, macrophages, neutrophils, or NK cells. It thus appears that the expression of MHC and leukocyte antigens by cells in the rat cornea is similar to that reported in the human, indicating that the rat may indeed provide a good model of human corneal transplantation.