11q23 translocation is the most popular chromosomal abnormality in infant leukemia. In adults, it is often encountered in non-Hodgkin's lymphoma (NHL). In this study, we analyzed the phenotypic and genotypic characteristics of 9 acute leukemic cell lines with 11q23 translocations and one with deletion of the 11q23 locus, nine of which were established by researchers in this group, together with 4 NHL cell lines with 11q23 translocations. All lines were considered to belong to the B-cell lineage at different stages. All 10 leukemic lines showed clonal rearrangement of the immunoglobulin heavy chain (IgH) gene: two corresponded to the B-precursor stage (CD19+, cytoplasmic mu-), while the other 8 corresponded to the pre-B stage (cytoplasmic mu+). All 4 NHL lines showed rearrangements of both the IgH and Ig kappa genes with three expressing surface Ig; specifically, mature B-cell phenotype. As for myelocytic-monocytic markers, at least one out of 4 antigens examined were positive in 8 of the 10 leukemic cell lines, while only one of the 4 NHL lines was reactive. There were essentially no clear phenotypic or genotypic differences between t(4;11) and t(11;19) cell lines, supporting the view that both diseases have similar clinicopathological characteristics. These cell lines are also valuable for cloning genes at the chromosomal breakpoints.