Various markers associated with the production of and response to interferons (IFNs) were studied in patients with either inactive rheumatoid arthritis (RA) or active RA, and in healthy subjects. The IFN markers assessed were serum and synovial fluid (SF) levels, the activity in peripheral blood leukocytes (PBL) of (2′-5′) oligoadenylate synthetase (OAS), and the production in vitro by PBL of IFN- α β in response to Sendai virus or Poly(I):Poly(C) as inducers, and of IFN-γ using PHA or Con A as inducers. IFN activity, tested by antiviral assays using two different cell lines, was not demonstrable in the serum of any patient with RA. The activity of (2′-5′) OAS in PBL, which may indirectly indicate exposure of leukocytes to IFN, was increased in RA compared with healthy subjects, more so in patients with inactive RA. The production of IFN- α β by PBL in response to Sendai virus was low in active RA but high in inactive RA, relative to production in healthy subjects. The production of IFN-γ by PBL in RA was lower than in healthy subjects, more so in active RA. Thus inactive RA (remission status) is marked by evidence of PBL having been influenced by interferon and being a state of augmented inducibility to an IFN- α β stimulus, whereas active RA is associated with low inducibility of PBL to an IFN stimulus, but no evidence of IFN production in vivo. Our findings underscore the relevance of interferon to remission/activity in rheumatoid arthritis.