SUMMARYK36.16 is an AKRH‐2kthymoma which expresses an aberrant H‐2Dd‐like allospecificity, does not have a detectable amount of the H‐2Kksyngeneic antigen and grows very easily in syngeneic mice. By DNA‐mediated gene transfer experiments, we were able to obtain transformed clones which do express the H‐2Kkmolecules and are rejected by AKR mice. Southern hybridization was performed to assess whether any gross changes had occurred in the K36.16H‐2Klocus or elsewhere in the MHC, which might explain the lack of H‐2K expression and/or the presence of the aberrant H‐2Dd‐like allospecificity. Specific H‐2 class I DNA probes were used to compare the K36.16 genomic DNA with normal AKR thymus DNA after digestion with a variety of restriction enzymes. After hybridization with the pH‐2IIa probe a 2.8 kb ‘Hind III’ fragment was identified in the K36.16 genomic DNA which is absent from AKR DNA. The pH‐2IIa probe detects the third, transmembrane and cytoplasmic domains of class I genes. Although these changes are indicative of MHC genome modifications it is not yet possible to link these specific Southern blot pattern variations with the pheno