A large number of continuous human leukemia cell lines have been established over the last three decades. Clearly, leukemia cell lines have become important research tools. Here, we have summarized the immunological, molecular and standard cytogenetic features of a panel of well characterized B cell precursor (BCP)-leukemia cell lines which were derived from patients with acute lymphoblastic/undifferentiated leukemia (ALL/AUL) or chronic myeloid leukemia (CML) in blast crisis. Following the recently proposed immunological EGIL classification, we assigned our panel of 27 BCP-cell lines to one of the following categories: B-I pro-B cell line; B-II common-B cell line; and B-III pre-B cell line. All cell lines express general B-lineage associated surface markers (HLA-DR, CD22, CD79a) being negative for surface immunoglobulin (Ig); the differences between the subgroups reside in expression of CD10 and cytoplasmic Ig. Several BCP-cell lines show the myelomonocytic cell-associated markers CD13 and/or CD33. These immunologically 'biphenotypic' BCP-cell lines are generally TdT+ CD10+ CD13+ CD19+ CD22+ CD34+ and carry the Philadelphia (Ph) translocation. The BCP-cell lines display surface receptors for interferon-gamma (CD119), interleukin-7 (CD127) and FLT-3 ligand (CD135). All BCP-cell lines examined have complex numerical and structural chromosomal alterations including translocations commonly seen in BCP-ALL such as t(4;11), t(9;22), t(11;19), t(12;21), and t(17;19) involving the fusion genes MLL-AF4, BCR-ABL, ENL-MLL, TEL/ETV6-AML1 and E2A-HLF, respectively. Besides the expected rearrangement of the Ig heavy chain receptor gene, several cell lines also have rearrangements of the T cell receptor genes beta, gamma or delta. While some BCP-cell lines express (aberrantly) myeloperoxidase at the mRNA level, most lines are negative in the immunological or cytochemical staining. Several large series documented the difficulty in establishing such BCP cell lines with success rates in the range of 10-20% (on average 15%). Still, since the establishment of the first bonafide BCP-cell line in 1974 (cell line REH), some 150 cell lines have been established of which, however, only a small percentage have been sufficiently well characterized and described. A higher success rate for immortalizing any given leukemia cell might depend on a closer emulation of the physiological in vivo microenvironment. The possibility to grow in vitro leukemia cells at will would represent ideal experimental systems permitting basic research and patient-specific investigations. In summary, the use of well-characterized BCP-cell lines provide unprecedented opportunities for studying a multitude of biological aspects related to normal and neoplastic B-lymphocytes.