Uveitis can be induced by systemic or intravitreal administration of endotoxin (lipopolysaccharide, LPS). In this study we correlated the expression of class II antigens (la in rat) of the Major Histocompatibility Complex (MHC), with this experimental model of uveitis, la antigen was detected by immunohistochemistry using the Avidin-Biotin-Peroxidase Complex (ABC) method and the monoclonal antibody OX6. la antigen was not expressed in normal eyes. However, la was expressed in the anterior uvea epithelial cells in all eyes with LPS induced uveitis.This study demonstrates that the ocular la expression is a localized process in the anterior uvea in response to systemic or intravitreal LPS. This response appears to be distinct from the action of LPS on macrophage la expression, where LPS has been shown to inhibit the induction of la antigen in macrophages by gamma interferon.