SUMMARYThe K36.16 thymoma expresses an H‐2Dd‐like allospecilicity as detected by the 34‐5‐8 monoclonal antibody directed against H‐2Ddusing immunoprecipitation techniques. This monoclonal antibody gives a negative result by radio‐immune assay but other anti‐Ddantibodies react positively with this tumour. These results confirm and extend previous observations and are attributed to probable gene‐conversion events possibly involving a donor gene from theQafTlaregion, the products of which cros