Peripheral blood lymphocytes from nine normal subjects and 40 patients with leprosy were pretreated in vitro with Concanavalin A (Con A). Cells from normal subjects pretreated for 24 hours showed consistent and effective generation of suppressive activity which inhibited mitogen induced transformation of autologous lymphocytes. Prolongation of Con A pretreatment to 40 hours resulted in maximal suppressive activity. Tuberculoid leprosy patients had lymphocytes in their blood which on 24 hour pretreatment with Con A exerted suppressive effects markedly greater than the maximal suppression noted with 40 hour pretreated normal lymphocytes. In contrast, untreated patients with polar lepromatous leprosy showed a decrease in suppressive activity which could not be altered by prolongation of Con A pretreatment: the loss of suppressive activity noted in this form of leprosy was restored during erythema nodosum leprosum.