Rheumatoid factor (RF) is contained in the serum of rheumatoid arthritis patients. Because of its high degree of specificity, the detection of RF by serologic test has proved to be useful in the clinical diagnosis and prognosis of rheumatoid arthritis. RF is an autoantibody of human immunoglobulin G (IgG). The most conventional serologic test for RF is the method dependent upon the agglutination of particles (e. g., latex and erythrocytes) which have been sensitized with human gamma-globulin. Assays for RF have been improved and more quantitative RF test have been reported (e. g., nephelometric immunoassay and turbidimetric immunoassay).