Two leukemic cell lines (697 and 207) were established from bone marrow cells obtained from children with ALL in relapse. These cell lines were positive for the common-ALL antigen (CALLA), the HLA-DR (i.e., Ia-like) antigen, and for cytoplasmic and surface IgM heavy chains. The lines were negative for other immunoglobulin heavy chains and light chains. The lines had elevated levels of terminal deoxynucleotidyl transferase enzyme and expressed surface antigens found on normal myeloid- macrophage cells (MMA) and on natural killer cells (HNK-1). A minority of cells in line 207 expressed the T-1, T-6, and Leu-1 antigens as detected by monoclonal antibodies. Line 697 was positive for Epstein- Barr virus (EBV), while line 207 did not possess EBV. Line 697 carried a marker chromosome (identified as a translocation between chromosomes 7 and 19), which was also patient's fresh leukemic cells. The leukemic origin of the cell lines was further indicated by their morphological, cytochemical, and immunologic similarity to the patients' leukemic cells. Phenotypically, both cell lines appear to be arrested in a transitional stage of development between pre-B and B cells and express surface antigens usually found on normal and fresh leukemic cells of non-B-cell lineages.