Rheumatoid factors are autoantibodies against Fc region of of IgG and is found in 70–80% of patients suffering from chronic rheumatoid arthritis, and are considered to be closely related to its pathological symdrome. Experimental animal models with spontaneous autoimmune diseases similar to those in humans, and animals with artificially-induced inflammation have been used to elucidate the mechanism of autoimmune diseases and to search for potential new remedies. A representative model animal of spontaneous autoimmune diseases is MRL/lpr mouse. As MRL/lpr shows high incidence of lymph node tumor, nephritis, angitis, and arthritis, this animal strain is useful for studies of the mechanism of human autoimmune diseases including rheumatoid arthritis. Autoantibodies found in MRL/lpr serum are IgG type rheumatoid factor, IgM type rheumatoid factor, anti-ssDNA antibodies, anti-dsDNA antibodies, and anti-Sm antibody, etc. This kit enables quantification and comparison of IgM type rheumatoid factor with a calibration curve using standard antibody preparation.