Amplified cellular genes in mammalian cells frequently manifest themselves as double minute chromosomes (DMs) and homogeneously staining regions of chromosomes (HSRs). With few exceptions both karyotypic abnormalities appear to be confined to tumour cells. All vertebrates possess a set of cellular genes homologous to the transforming genes of RNA tumour viruses, and there is circumstantial evidence that these cellular oncogenes are involved in tumorigenesis. We have recently shown that DMs and HSRs in cells of the mouse adrenocortical tumour Y1 and an HSR in the human colon carcinoma COLO320 contain amplified copies of the cellular oncogenes c-Ki-ras and c-myc, respectively. Both DMs and HSRs are found with remarkable frequency in cells of human neuroblastomas. We show here that a DNA domain detectable by partial homology to the myc oncogene is amplified up to 140-fold in cell lines derived from different human neuroblastomas and in a neuroblastoma tumour, but not in other tumour cells showing cytological evidence for gene amplification. By in situ hybridization we found that HSRs are the chromosomal sites of the amplified DNA. The frequency with which this amplification appears in cells from neuroblastomas and its apparent specificity raise the possibility that one or more of the genes contained within the amplified domain contribute to tumorigenesis.