Several unusual chromosome structures have been described in drug-resistant cell lines and in certain tumours. These structures include elongated homogeneously staining regions (HSRs), small extrachromosomal paired chromatin bodies (double minutes, DMs) and abnormally banded regions (ABRs) with strong but anomalous band patterns. There is evidence that these are alternative forms of gene amplification, with HSRs breaking down to form DMs, and DMs integrating into the chromosome to generate HSRs and ABRs. Recently, it was demonstrated that, compared with several normal and leukaemia human cells, DNA sequences representing the human homologue of the onc gene of the avian myelocytomatosis virus (MC29), the so-called c-myc gene, were amplified in HL-60 cells. This is a human promyelocytic leukaemia cell line established in the laboratory of one of us (R.C.G.) at the National Cancer Institute (Bethesda, Maryland) in 1977, and widely used for studies on myeloid and monocytic differentiation. Amplification of the gene was present in primary leukaemic cells of the patient, and DMs were noted in some of these cells as well as in early passages of the HL-60 line. No structure resembling HSRs or ABRs were noted in karyotypic studies at this early stage and there were no alterations involving the long arm of chromosome 8 (8q), to which the c-myc gene has recently been mapped. We have now re-examined the karyotype of the HL-60 line, using cells frozen at various times during its continuous passage at the Wistar Institute (Philadelphia, Pennsylvania) to look for chromosomal abnormalities that might be associated with the amplification of c-myc. We find that, beginning in 1979, HL-60 cells at the Wistar Institute no longer had DMs, but did show an abnormal 8q+ chromosome, replacing a normal chromosome 8, and representing an ABR reflecting the site of myc gene amplification.