BACKGROUND Previously, we identified a regulatory rheumatoid factor (regRF), the production of which provides rats with resistance to collagen-induced arthritis (CIA). Immunization with conformers of IgG Fc fragments carrying epitopes specific to regRF reduces symptoms of CIA. The aim of this study was to determine whether there is a link between regRF levels and rheumatoid arthritis (RA) activity in humans in order to assess the potential of regRF as a therapeutic biotarget in RA. The variability of rheumatoid factor (RF) specificities present in the blood of RA patients was also studied. METHODS The regRF were studied in RA patients with active disease and in remission. Variability in the specificities of RF associated with RA was studied by concurrent inhibition of RF latex fixation by variants of modified IgG. RESULTS Patients in remission had regRF levels higher than in healthy subjects. The regRF in remission was characterized by tight binding to its antigen, as in healthy subjects. The regRF levels in patients with active RA varied dramatically, and regRF binding to its antigen was weak. The exacerbation of Still's disease coincided with low regRF levels and affinity, while an improvement in patient condition was associated with an increase in regRF levels and affinity. The RF specific to RA, which was detected by the RF latex-fixation method, was a nonhomogeneous population of antibodies that included RF to lyophilized IgG, to IgG immobilized on polystyrene, and to rabbit IgG. CONCLUSIONS Stimulating regRF production might enable improved RA therapy.