An Alu polymerase chain reaction (PCR) probe specific for chromosome 11 prepared from the somatic cell hybrid J1 was used to analyze karyotypes of eight patients with acute monocytic leukemia (AML-M5). Chromosome painting confirmed the t(9;11) in one patient and a der(1)t(1;6)t(6;11) in another and allowed the identification of a complex rearrangement involving chromosomes 9, 11, and 17, previously classified as del(11)(q23), in a third patient. An analysis of five patients with AML-M5 and a normal karyotype did not detect abnormalities of chromosome 11. The usefulness of chromosome painting combined with in situ hybridization with probes previously located on particular chromosomes is emphasized.