Oral lichen planus (OLP) is a common inflammatory condition of the oral mucous membranes which affects between one and two percent of the general population. In accordance with the protracted clinical course of OLP and its association with known auto-immune diseases, the level of self-tolerance is questionable and possibly diminished in patients with this disorder. Normal suppressor T lymphocyte function is reputedly an essential element in the maintenance of self-tolerance, and deficient cell-mediated suppressor activity is implicated in the pathogenesis of auto-immune diseases. For assessment of in vitro cell-mediated suppressor activity in OLP, peripheral blood mononuclear cells (PBMC) from ten patients with OLP and from 11 control subjects were activated with the plant mitogen concanavalin A (Con A), followed by co-culture with autologous responder cells. The ability of irradiated Con A-activated cells to suppress the proliferation of Con A-stimulated responder cells was determined. Con A-induced suppressor activity of PBMC in the OLP patients was significantly less than that in control subjects (p = 0.001). Results of the present investigation complement previous in vitro findings which provided indirect evidence of deficient cell-mediated suppressor activity in OLP, particularly a decreased proportion of circulating CD4+CD45RA+ lymphocytes and reduced Con A-stimulated PBMC proliferation. The depressed Con A-induced suppressor activity of PBMC in the OLP patients provides direct evidence of deficient in vitro cell-mediated suppressor function in OLP, and suggests that defective cell-mediated suppressor circuits and reduced self-tolerance may be involved in the pathogenesis of this disorder.