Activation of suppressor cells by the autologous mixed lymphocyte reaction (AMLR) was studied in patients with juvenile rheumatoid arthritis (JRA). Isolated OKT4 cells were used as responders and irradiated whole non-T cell preparations, non-adherent non-T cells (B and null cells), adherent non-T cells (monocytes), and B cells preactivated by Staphylococcus aureus Cowan I, as stimulators in the AMLR. Suppression was assessed by a reduction of pokeweed mitogen stimulated immunoglobulin synthesis by lymphocytes of healthy donors upon addition of AMLR-activated OKT4 cells. The results of this study suggest normal activation of OKT4 suppressor cells in the AMLR of patients with JRA.