Ferritin in sera and in synovial fluids were determined by method of immunoradiometric assay (IRMA) and of radioimmunoassay (RIA). These methods have been proved to be reproducible. The coefficient variation was 7.4 to 9.9% in IRMA and 5.6 to 8.0% in RIA. There were a good correlation between methods of IRMA and of RIA (r=0.989). The mean recovery rate of ferritin in serum was 101.2% in IRMA and 101.0% in RIA, respectively. The determination of serum ferritin revealed a 16% reduction in average after preservation at -20℃ during over 100 days. The levels of serum ferritin in healthy controls were 131.4±52.1 ng/ml in 34 males and 58.4±39.9 ng/ml in 38 females. The sex differences were statisticalIy significant (p<0.01). Ferritin levels were shown to be below 30 ng/ml (the state of iron deficiency) in 2 of 34 (5.9%) males and in 8 of 38 (21%) females of healthy controls. The levels of serum ferritin in patients with rheumatoid arthritis (RA) were 337.0±293.4ng/ml in 10 males and 181.4±329.9ng/ml in 25 females. The serum ferritin in RA was significantly higher than in healthy controls of both sexes. The levels of ferritin in synovial fluids of patients with 14 RA and with 12 osteoarthritis were 2894±3017 ng/ml and 1429±1005 ng/ml, and no differences were observed in both groups.