SummaryExpression of H‐2 antigenic specificities on K36, a spontaneous leukaemia originating from AKR (H‐2k) mice, was studied by serology and immunochemistry. Two ascites lines of the tumour, as well as a tissue culture adjusted and cloned tumour line, were used in these studies with similar results being obtained.K36 expresses on its cell surface D‐region encoded H‐2K antigens but does not express K‐region encoded H‐2K alloantigens. It also expresses on its cell membrane, H‐2 specificities of foreign haplotypes not present on normal AKR lymphoid cells. The molecular basis of the H‐2Ddspecificity on K36 (H‐2kwas analysed by immunoprecipitation and polyacrylamide gel electrophoresis. The specificity was shown to be present on a glycoprotein of apparent molecular weight 45,000. However, antisera against the H‐2Ddprivate specificity (H‐2.4) precipitate additional glycoprotein of 45,000D and also 70,000D.In tryptic peptide maps of the isolated 45,000D fraction precipitated by anti‐H‐2.4 serum from radiolabelled K36 glycoprotein, all H‐2Ddspecific peptides were present in the same quantitative ratio. This is consistent with the structural identity of the foreign H‐2Ddfrom the K36 tumour with normal H‐2Ddand supports the hypothesis of a regulator system controlling theH‐2allelism. Under certain circumstances such a system could cause suppression of one and derepres