Synovial fluids from patients were examined, including 15 cases of rheumatoid arthritis (RA); 9 cases of sero-negative arthropathy (3 cases of ankylosing spondylitis, 6 cases of reactive arthritis); 1 case of gout arthritis; and 5 cases of osteoarthritis. The synovial fluids contained 330-72600 white blood cells/mm3, 9-97% of which were granulocytes, in RA patients. Other physical features of RA fluids were: straw green color, cloudy or unclear; moderate viscosity; fair or poor mucin clotting; lack of crystals and bacteria. Synthesis of the above may contribute to the diagnosis of RA. We also found that IgG, IgA and IgM of serum and IgG of synovial fluids in 15 cases of RA were higher than in the other arthritis groups; the IgM-RF positive rates of RA synovial fluids and sera were 20% and 35.7% respectively, while all were negative in the other groups. Immunoglobulins and complement levels in the synovial fluids of all the tested arthritis cases were about half of those in the serum.