We have previously determined that the amplified DNA present in the HL60 promyelocytic leukaemia cell line contained 70 kb of continuous DNA sequences around the c-myc gene. In the work presented here we have further defined the HL60 amplicon and find it to be of the order of 160 kb and to contain a large region of DNA from chromosome 8q24 that is located at least 260 kb telomeric to the c-myc gene, joined to the 70 kb of DNA from the c-myc gene region. The novel chromosome 8 DNA coamplified sequences are not lost during multiple passage of HL60 cells since the composition of the chimeric amplicon is the same in both early passage HL60 cells containing only double minutes (DMs) and late passage isolates containing homogeneous staining regions (HSRs) at different chromosomal locations. This shows that the HL60 HSRs found in late passage cells are not generated anew but are directly derived from the precursor DMs. The HL60 cells contain two copies of chromosome 8, each with different polymorphic markers. Both these chromosome 8 homologues retain a c-myc gene as well as the region containing the coamplified DNA sequences indicating that the HL60 amplicons were not generated by simple DNA deletion. The constraint to maintain the novel DNA sequences coamplified with c-myc gene DNA suggests that these sequences may play some role in maintaining the growth potential and/or differentiation capacity of the HL60 cells.