AbstractDifferent human IgM rheumatoid factor (IgM RF) idiotypes have been described defined by polyclonal rabbit anti‐idiotypic antibodies. These antisera do not allow clear genetic analysis of the idiotypic determinants, be they cross‐reactive or private. Therefore, we tried to obtain a set of monoclonal anti‐idiotypic antibodies directed against RF idiotypes. Purified IgM RF serum from a patient with classical rheumatoid arthritis was used to immunize BALB/c mice. The spleen cells were fused with Sp 2/0 Ag 14, a nonsecreting mouse myeloma cell line, and a hybrid producing monoclonal anti‐idiotypic antibody was selected. The mouse antibody, an IgG1x, reacts with an identical or similar determinant located on (or close to) the binding site of all tested monoclonal or polyclonal IgM RF from totally unrelated patients with Waldenströms's macroglobulinemias or rheumatoid arthritis. The monoclonal antibody also reacts with 2 rheumatoid arthritis patients' IgG RF and with a low proportion of normal polyclonal IgM without detectable RF activity. An hypothesis is proposed to explain the existence of a such highly conserved determinant on RF i