Three sublines of the human promyelocytic leukaemia cell line HL60 have been isolated. Histochemical, karyotypic and immunological analyses have confirmed the identity of the three sublines as HL60. However, the extent to which c-myc homologous DNA sequences are amplified in the genomes of these cells varies from 4- to 30-fold. Moreover, the relative abundance of c-myc-related RNA sequences in uninduced cells of each subline is directly proportional to the gene copy number. It is concluded that, while high levels of the c-myc gene and its transcripts may have had a role in the establishment of HL60 cells, these are not required for maintenance of the HL60 phenotype. However we cannot rule out the possibility that a low level of amplification and enhancement of transcription has a role in maintenance of this phenotype.