Post-transplant lymphoproliferative disorders (PTLD) are well-known complications of iatrogenic immune deficiency and are thought to result from the proliferation of B cells infected by the Epstein-Barr virus (EBV). Some large cell lymphomas occurring in the general population carry a rearrangement of the Bcl6/Laz3 zinc-finger-encoding gene. 15 EBV-associated PTLD were tested for the presence of Bcl6/Laz3 rearrangements by Southern blot analysis using two specific probes (F370, F372). One out of 15 cases displayed a rearranged band independent of the germline one. In contrast, 10 lymphoblastoid cell lines and one lymphoblastoid cell line passaged in an SCID mouse carried only germline alleles of Bcl6/Laz3 after Southern blot hybridization. This indicates that genetic abnormalities may also play an important role in the development of some PTLD.