We have found a single 4p+ chromosomal abnormality, 46,XX, -4, +der(4)t(3;4)(q13.3;p16), in a patient with an unusual B cell leukemia of mature phenotype characterized by a high white cell count, tartrate-resistant acid phosphatase-positive malignant cells, splenic white pulp proliferation, and a serum IgM monoclonal gammopathy. The malignant cells were characterized by surface expression of CD19 (B4), CD20 (B1), IgM, IgD, kappa, and HLA-DR. They were weakly positive for CD21 (B2) and negative for CD25 (interleukin-2 receptor). The malignant cells also showed clonal rearrangement of the immunoglobulin heavy chain and kappa light chain genes. A cell line, designated HCLW-3B, was derived from unstimulated peripheral blood obtained during the leukemic phase and was found to contain the same 4p+ chromosomal abnormality as well as genomic sequences of the Epstein-Barr virus nuclear antigen. A somatic cell hybrid constructed from HCLW-3B containing the derivative chromosome 4 was used to confirm that chromosome 3q was the source of the translocated material. The availability of a cell line which is clonally derived from the patient's circulating leukemia cells should permit further characterization of this translocation at the molecular level.