We have studied the immunoregulatory function of T8+ (suppressor/cytotoxic) and Leu3a+ (inducer/helper) T cells from rheumatoid arthritis (RA) patients by measuring the effect of these T-cell subpopulations on the generation of immunoglobulin-secreting cells by normal allogeneic B cells after stimulation with pokeweed mitogen (PWM) in vitro. When T8+ or Leu3a+ cells from blood or synovial tissue from nine patients were substituted for T8+ or Leu3a+ cells, respectively, from normal blood mononuclear cells (MNC), RA T8+ cells showed an increased suppressor activity, whereas RA Leu3a+ cells were, except for one patient, weak augmentors. Unreplaced normal MNC and MNC replaced with allogeneic normal T-cell subpopulations responded equally to PWM. When T8+ plus Leu3a+ cells from the same patient replaced normal T cells, high B-cell responses were detected. Normal T8+ plus Leu3a+ cells generally supported the response to a lower degree. Substitution with two allogeneic T-cell subpopulations did not result in a B-cell response to PWM. Thus, whereas RA T8+ seemed to be strong suppressors and RA Leu3a+ cells weak augmentors by themselves, together they are possibly able to generate a B-cell stimulatory potential that might be of pathogenetic significance in the patients.