Peripheral blood lymphocytes (PBL) from 14 patients with rheumatoid arthritis (RA) produced increased amounts of interleukin (IL) (p less than 0.05) as measured in a mouse thymocyte assay and showed enhanced proliferation in response to an IL containing supernatant (p less than 0.05) when compared with 9 age matched controls. Both enhanced IL production (p greater than 0.01) and responsiveness (p less than 0.002) were seen exclusively in a subgroup of 7 patients with a recent onset or exacerbation of their disease. PBL from RA patients with equally active disease which had been unchanged for more than 6 months produced and responded to IL normally. There was a direct correlation between IL production and responsiveness (r = 0.69, p less than 0.005). These 2 distinct IL abnormalities appear to reflect disease initiating or exacerbating factors in RA.