Latency of Epstein-Barr virus infection may be generated by surviving immortalized B cells or by continuous re-infection. EBV-positive B-cell tumors have been found following bone-marrow transplantation (BMT) and were of donor type in the few cases investigated. We established a B-cell line from the bone marrow of a patient in complete remission following allogeneic BMT for aplastic anemia 18 months post-grafting. Differences in sex and isoenzymes allowed an exact determination of chimerism in our case. While the patient showed persistent complete chimerism of all cell lineages, cells grown in culture were of recipient type. They expressed B-cell markers, showed a monoclonal rearrangement of the immunoglobulin genes and carried EBV-associated antigens. As direct preparations of cells from the patient did not contain detectable recipient-type cells, it appeared likely that small numbers of EBV-transformed B cells of the recipient survived for long periods in this patient. For the development of secondary B-cell neoplasms in vivo, additional patho-physiological steps like severe graft versus host disease or T-cell suppression are obviously required because the patient was still free of lymphoma 3 years post-grafting.