Résumé : Anti‐idiotype (Id) antibodies against anti‐a1 rabbit allotype antibodies were produced in rabbits and mice by immunization with polyclonal rabbit anti‐a1 allotype or with a monoclonal murine anti‐a1 allotype antibody, respectively. These antibodies recognize an interspecies cross‐reactive idiotope on anti‐a1 allotype antibodies. However, these anti‐Id antibodies also react with various subclasses of human IgG. In multiple systems, it was shown that these anti‐Id antibodies interact with the Fc fragment of human IgG and, therefore, exhibit a rheumatoid factor‐like activity. These results support the hypothesis that γ‐globulin may not be the only stimulus to clones producing rheumatoid factors and that perhaps other antibodies, especially anti‐Id antibodies, also bind γ‐globulin secondarily. Copyright © 1984 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim