Orosomucoid, haptoglobin, PCA‐soluble glycoproteins, and sialic acid have been determined in serum and synovial fluid in rheumatoid arthritis. The increase in the serum content of sialic acid in rheumatoid arthritis is largely due to an increase in orosomucoid and haptoglobin. There is no indication that these glycoproteins arise from peripheral production in the inflammatory synovial tissue. The presence in synovial fluid of other glycoproteins of tissue origin, however, can not be excluded.