The chromosome composition of the human leukemic cell line, K562, deposited at the American Type Culture Collection (ATCC) (CCL 243), was studied and further compared with those reported by other laboratories. In CCL 243, the modal karyotype had 55 normal and 14 constitutive marker chromosomes and occurred in nearly 50% of the cultured cell population. Other variant karyotypes were observed either at a low frequency in coexisting minor cell types or only once in all other analyzed metaphases. Normal chromosome #9 and the chronic myeloid leukemia (CML) specific Ph/t(9;22) were absent. It appears that karyotypes of most K562 cultures distributed world-wide are similar to each other, with the stable modal chromosome number, and therefore, the modal karyotype presented here should be useful in karyotypic identification of a cell line.