The HL60 cell line was established in 1977 from a patient with acute myeloid leukaemia. The cells largely resemble promyelocytes but can be induced to differentiate terminally in vitro. Some reagents cause HL60 cells to differentiate to granulocyte-like cells, others to monocyte/macrophage-like cells. The HL60 cell genome contains an amplified c-myc proto-oncogene; c-myc mRNA levels are correspondingly high in undifferentiated cells but decline rapidly following induction of differentiation. These features have made the HL60 cell line an attractive model for studies of human myeloid cell differentiation. This review summarizes the major properties of HL60 cells, describes some aspects of the regulation of gene expression in differentiating HL60 cells, including a novel interaction between transcriptional and post-transcriptional controls, and discusses the possible involvement of c-myc in proliferation and differentiation.