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HLA-DR is a Class II histocompatibility antigen which has recently been reported to occur in increased amounts in dermal and oral lichen planus. Its expression in epithelium wits previously thought to be limited to Langerhans cells. In this study, formalin-fixed sections of oral lichen planus and a variety of other oral mucosal lesions were stained with a monoclonal antibody to HLADR using an indirect immunofluorescence lest. The expression of HLA-DR by keratinocytes did not appear to be a specific marker for lichen planus as demonstrated by the reactivity of the epithelium in a variety of lesions of the oral mucosa. If HLA-DR expression ix considered to be an inducible function, perhaps in response to γ interferon secreted by activated lymphocytes, then keratinocyte reactivity would not be expected to occur only in lichen planus. [ABSTRACT FROM AUTHOR]