Serum glycosaminoglycans were studied in patients with rheumatoid arthritis by means of electrophoresis. Dismucopolysaccharidemia, which was observed in these patients, was manifested in alteration in the ratio of hyaluronic acid and chondroitin-4-sulphate and also in occurrence of heparin associated with protein and chondroitin-6-sulphate in blood serum. The latter fractions of glycosaminoglycans are not usually found in blood of healthy persons. The alteration in the ratio of fractions of glycosaminoglycans was accompanied by a decrease in content of hyaluronic acid in synovial tissue of the joints impaired, by an increased excretion of the substance with urine and also by the increased activity of hyaluronidase in blood serum, articular fluid and in synovial tissue. The alterations in the composition of blood glycosaminoglycans, which correlated with the clinical manifestations of the disease, suggest that impairments of metabolism of the carbohydrate-containing biopolymers are involved in the genesis of rheumatoid arthritis.