Ia antigen (class II antigen) is a histocompatibility antigen that foreign peptides associate with, before antigen presentation to T cells and subsequent triggering of the CD4 T cells. Although corneal epithelium is normally Ia negative it may become Ia positive under abnormal circumstances but the functional significance of this is uncertain. In this study the expression of Ia antigen on corneal epithelium of mice during in vivo primary and secondary herpes simplex keratitis and the in vitro accessory function of corneal epithelium in the presentation of herpes simplex virus (HSV) antigen to in vivo HSV primed T cells were evaluated. Whole mount preparations of corneal epithelium were found to express Ia antigen on days 3, 5, and 7 following corneal inoculation with live HSV. The intensity of the Ia expression was greater in non-immune mice on day 7 after corneal inoculation compared with immune mice. A cellular suspension of corneal epithelium induced HSV primed T cells to proliferate in the presence of HSV antigen. Induction of Ia antigen on corneal epithelium during herpes simplex keratitis may functionally expand the population of antigen presenting cells in the cornea and contribute to T cell activation.