The expression of the Class II products DR and DQ on human gingival epithelium was examined using immunofluorescence and immunoperoxidase staining. Differential expression of Class II antigens was seen in chronic gingivitis in adults, with T6(+) DR(+) cells being more numerous than T6(+) DQ(+) cells. The periodontopathic organismFusobacterium nucleatum(FN) induced DQ expression on Langerhans' cells (LC) duringin vitroexplain culture of gingival tissue. This effect was mimicked by endotoxin (LPS) fromF. nucleatumand byE. coliLPS. These results indicate that differential expression of Class II products, a feature of chronic gingival inflammation, may result from the action of LPS on gingival LC.