The body weight gain of the rat, intestinal absorption of B_<12>, and B_<12>-binding capacity of rat's stomach were studied by using ^<57>Co-cyanocobalamin after oral administration of synovial fluid with joint diseases in order to investigate the effect of synovial fluid of rheumatoid arthritis on the B_<12> metabolism in the rat. No change was found in body weight gain of rats by administration of synovial fluid. The ^<57>Co- B_<12> uptake in liver, kidney, stomach, spleen, particularly, in pituitary gland and adrenal of the rats treated with oral ingestion of synovial fluid of rheumatoid arthritis was lower than that of osteoarthritis. In the B_<12> recovery represented by fecal excretion of B_<12> was found to be higher in rheumatoid arthritis than that of osteoarthritis. The binding capacity of rat's stomach treated with the synovial fluid of rheumatoid arthritis was lower than that with osteoarthritis and control group.