The sulfhydryl groups are chemically active parts of the protein molecule and participate in many important biochemical and metabolic processes. The present material comprised 132 patients with rheumatoid arthritis, 104 patients with other diseases and 44 controls. The patients with rheumatoid arthritis had decreased serum sulfhydryl group levels, the decrease being greater in patients with active disease. Changes in the activity of rheumatoid arthritis were correspondingly followed by changes in the serum sulfhydryl group levels. The serum sulfhydryl group levels diminished significantly with advancing age. Similarly decreased serum sulfhydryl group levels could also be observed in many other diseases.