The rheumatoid factors constitute one of the major autoantibodies in rheumatoid arthritis (RA). In this study, for the quantitative determination of the RF IgM class, in serum and synovial liquid of patients with RA and JRA, we used the hemagglutination principle and both liquid and lyophilized reagents. The results we obtained demonstrated the superiority regarding the stability, the reproducibility and the possibility of micromethod standardization with lyophilized reagent.