Rheumatoid factors (RF) are the most common type of functional antibodies among naturally occurring human monoclonal IgM proteins. A large subset of these autoantibodies use structurally homologous light chains of the kIII subgroup, which bear the 6B6.6 cross‐reactive idiotype (CRI). Although antibody binding activity requires both heavy and light chains, information about the heavy chains used by these autoantibodies is limited. To investigate these proteins, the murine monoclonal antibodies, 5–14 and 6–10, were generated by immunization with the heavy chains of the 6B6.6 CRI‐positive RF, COR and LEW. These antiidiotypic antibodies reacted with 8 of 11 autoantibodies that coexpressed the 6B6.6 CRI. All 8 RF had heavy chains from the V_H4 gene family, as assessed by reactivity with a V_H4‐specific primary sequence‐dependent antibody. The same RF were also identified by the previously described murine monoclonal antiidiotype, LCI. Further experiments revealed that the LCI antibody delineates a subfamily of V_H4 heavy chains that is preferentially used in KIII‐6B6.6 CRI‐positive IgM‐RF. The cumulative data suggest that 13–22% of RF express both the KIII‐6B6.6 and V_H4‐LC1 CRI. These findings document that RF autoantibody activity requires specific V_L‐V_H pairing, and that a subset of idiotypically related V_H4 heavy chains is commonly expressed in disease‐associated monoclonal IgM‐RF.