Chromosome 11 band q23 is commonly involved in nonrandom chromosomal translocations in hematpoietic malignancies, especially in infant acute leukemias. By using pulsed-field gel electrophoresis (PFGE) with restriction endonuclease digests of DNA from both a leukemia cell line (RS4;11) bearing the t(4;11)(q21;q23) and from human/hamster hybrid cells, we have been able to construct a detailed restriction map of the chromosome 11q23 region and have localized the t(4;11) chromosome 11 breakpoint to a region located approximately 200 to 230 kb telomeric to the CD3γ region and approximately 580 kb centromeric to the PBGD gene. PFGE analyses of DNA from clinical leukemia specimens and cell lines indicated a tight clustering of breakpoints in all eight t(4;11) acute leukemias studied. These data strongly suggest that discrete genetic loci are interrupted on both chromosomes 4 and 11 in a manner likely to be critically involved in the pathogenesis of t(4;11) acute leukemias. To our knowledge, these results represent the first evidence of breakpoint clustering in t(4;11) acute leukemias. In contrast to t(4;11), other 11q23 abnormalities studied to date have frequently shown evidence for alternative breakpoint sites in 11q23.