AbstractAnti‐idiotypes produced against monoclonal anti‐H‐2 antibodies have been used to examine idiotope sharing among a panel of anti‐major histocompatibility complex (MHC) antibodies detecting the same and different specificities. Both xenogeneic anti‐100‐30 and anti‐3‐83, which did not react with predominant idiotypes in conventional alloantisera, detected shared idiotopes on 3‐83 and 100‐30 as well as on 2 other monoclonal anti‐H‐2Kkantibodies. All 4 monoclonal antibodies recognized the same epitope cluster on the Kkmolecule and detected the same serological specificity, H‐2.5. In contrast, syngeneic and allogeneic anti‐idiotype produced against the same monoclonal antibodies did not detect these cross‐reactive determinants. In several instances, xenogeneic anti‐idiotype reacted with anti‐MHC antibodies which recognized distinct H‐2 determinants, suggesting that anti‐MHC antibodies detecting the same or different specificities may share idiotypic determinants. These reagents may be useful pro