Contradictory results have been reported concerning the secretion of interleukin-1 (IL-1) and interleukin-2 (IL-2) by mononuclear cells of rheumatoid arthritis (RA) patients. In the present study, peripheral blood mononuclear cells from 18 RA patients were stimulated in vitro to produce IL-1 and IL-2 and compared with monocytes of age-matched healthy control subjects. Endotoxin-stimulated monocytes of RA patients produced normal amounts of IL-1 compared with healthy controls (P = 0.5), whereas T-lymphocytes from the same patients produced decreased amounts of IL-2 compared with control T-lymphocytes (P less than 0.01). There was no difference in IL-1 or IL-2 production by mononuclear cells from patients with active or inactive disease. These findings could not be explained by concurrent therapy, and support the notion that defective immunoregulatory T-cell functions are involved in the pathogenesis of RA.