Rheumatoid arthritis is a systemic chronic inflammatory disease characterized by development of severe destruction of the joints caused by uncontrollable activation of cellular immunity. Activated T-lymphocytes synthesize interleukin-2 (IL-2) and specific membrane soluble receptors (sIL-2r) for this cytokine. The level of sIL-2r release is proportional to the number of molecules expressed on the membrane of activated immunocompetent cells. The data are provided for 59 patients with a significant RA diagnosis according to criteria of the American Rheumatology Board. Serum sIL-2r level was measured by enzyme immunoassay. It was found higher in RA patients than in controls (p < 0.001). Concentrations of sIL-2r increased with activation of RA. It is suggested that determination of sIL-2r extends potentialities of laboratory diagnosis of RA activity which is important for evaluating effectivity of the treatment and RA prognosis.