Rheumatoid arthritis is a chronic inflammatory systemic disease of young or middle aged adults, characterized by destructive and proliferative changes in synovial membrane, periarticular structure, skeletal muscle and perineural sheaths. Eventually, joints are destroied, ankylosed and deformed. Moderate anemia is occured frequently in rheumatoid arthritis patients, and these patients show abnormalities of iron metabolism such as lower serum iron concentration and occurance and distribution of iron in the synovial membrane. It has been suggested that the mechanism of iron deposits in rheumatoid arthritis is continuous oozing of blood from vascular granulation tissue into the synovial cavity and this lead the patient to anemia. We collected samples from serum and knee joint fluid in 21 cases of rheumatoid arthritis for chemical estimation of the ferritin concentration by radioimmunoassay, and compared with that of osteoarthritis. The following results were obtained: 1. The mean hemoglobin concentration(11.4gm%) in rheumatoid arthritis was lower than osteoarthritis(13.4gm %). 2. The mean serum ferritin concentration(118. 4ng/ml) in rheumatoid arthritis was lower than osteoarthritis(135. 6ng/ml), on the contrary in synovial fluid rheumatoid arthritis(279.8ng/ml) showed higher than osteoarthritis(190. 4ng/ml). 3. The mean ratio of synovial fluid ferritin on serum ferritin was 2. 36 in rheumatoid arthritis, in contrast with l. 4 in osteoarthritis. There was significant correlation between the ferritin concentration in synovial fluid and serum. 4. Serial check of ferritin concentration in synovial fluid during treatment would be thought meaningful criteria for determination of progress.and effectiveness of treatment.