Mononuclear cells from a 44-year-old patient with acute myeloid leukemia (AML) gave rise to a spontaneous permanent cell line cultured in suspension. The cell line was shown to be positive for Epstein-Barr virus nuclear antigen (EBNA). As expected, its composite phenotype was of B-cell type with B-cell antigens (CD 20, CD 21) and with monoclonal surface IgM of kappa type, but without detectable IgM secretion. Surprisingly, identical monoclonal rearrangements of the immunoglobulin heavy chain (JH) sequences could be demonstrated in the uncultured bone marrow AML cells and in the cell line that also had kappa light chain gene rearrangement. This is the first case to our knowledge of an EBNA positive B-cell line with identical monoclonal Ig heavy chain rearrangement as detected in myeloblastic leukemia cells.