The frequency and concentration of specific factors binding beta 2 microglobulin were investigated in sera and synovial fluids of patients and in sera of normal controls. High anti-beta 2 m activity was detected in the sera of adult RA patients, particularly in those of with extra-articular disease. Similarly, anti-beta 2 m was present in the synovial fluids of RA but not of osteoarthrosis patients. Both the binding of anti-beta 2 m activity to the Sepharose staphylococcal protein A and its elution position in the second 'IgG' peak after Sephadex G-200 gel filtration suggest the antibody nature of the activity. The possibility of differences not only in titre but also in the specificity of heterologous and homologous anti-beta 2 m antibodies are discussed.