Concanavalin-A-induced suppressor cell activity was investigated in 63 patients with a definite diagnosis of juvenile rheumatoid arthritis. Peripheral blood lymphoid cells from these patients did not have the same ability as cells from normal individuals to suppress the proliferative response of autologous cells, responding to phytohaemaglutinin, Candida albicans antigen, or allogeneic cells. No correlation was found between suppressor activity, disease activity, or number of joints involved. Nor was there any significant association between decreased suppressor cell activity and HLA-A, -B, -C, -D antigens, although there was a tendency towards association between decreased suppressor cell activity and HLA-B27.