SummaryThe inbred mouse strains BN/a and BN/b have haplotypeH‐2bpcharacterized by H‐2.33 and lacking any other private specificity known in inbred strains. Two transplantable B cell leukaemias which originated in BN/a and BN/b mice treated with anti‐lymphocyte globulin were tested serologicaly for H‐2 antigens. Tests during passages 169‐181 revealed several quantitatively different reactions with sera against public specificities, some of these being due to Ia antibodies. No change in expression of the private specificities was seen. On the other hand, during the later passages (239‐258) a number of qualitative differences were seen, the most remarkbale being the loss of H‐2.33 and gain of H‐2.4, 31. The overall serological pattern of cells resembled that ofH‐2drather than ofH‐2bphaplotyes and this was confirmed by absorption tests. The changes reported here may be due to alterations of repression and derepression pattern of the presumed multiple structuralH‐2genes