We have studied peripheral blood mononuclear cells obtained from 24 patients with acute or chronic active hepatitis to determine if there was an abnormality in concanavalin A-induced suppressor cell activity compared to control subjects. Suppressor cells were generated by preincubation of the mononuclear cells with a mitogenic concentration of concanavalin A (6 mug/ml) for 48 hr followed by treatment with mitomycin C and alpha-methyl mannoside. Suppressor cell activity was assessed in second cultures by inhibition of concanavalin A-stimulated blast transformation of fresh allogeneic lymphocytes. Concanavalin A-stimulated suppressor activity was not elicited in mononuclear cells from the majority of patients with chronic active hepatitis in contrast to patients with acute hepatitis or acute inflammatory diseases and controls (P < 0.001). This finding was demonstrable in chronic active hepatitis patients in remission and relapse, both on and off prednisone therapy, and varied considerably during the course of the disease. The extent of liver injury was not related to the measured suppressor cell activity. These studies suggest that in chronic active hepatitis, a disease in which the host immune response may be involved, there appears to be a defect in concanavalin A-stimulated suppressor cells.