Studies of the growth regulation, differentiation and transformation of myeloid cells have been greatly facilitated by the availability of a variety of hematopoietic growth factor-dependent cell lines. These cell lines have been isolated from long-term bone marrow cultures and myeloid tumors using interleukin 3 (IL-3) as a growth factor. Using growth factor-dependent cells, it has been shown that growth regulation by IL-3 involves binding to a high-affinity receptor of 140 Kd and activation of tyrosine phosphorylation. IL-3 binding is associated with a number of cellular responses which are required for maintenance of viability, including induction of transcription of the c-myc and ornithine decarboxylase (ODC) genes. In addition, IL-3 regulates the expression of transcription of the gamma T cell receptor locus. The properties of the IL-3-dependent lines are consistent with the hypothesis that they are transformed in their ability to terminally differentiate. In some of the cell lines, this transformation may terminally differentiate. In other of the cell lines, this transformation may be due to the altered expression of the c-myb gene. In other cell lines, transformation is associated with the activation of the expression of a novel gene, termed Evi-1, of the zinc finger family of transcriptional factors. Comparable transformation of erythroid lineage cells is speculated to be due to the activation of the expression of another novel gene termed spi-1. These studies have emphasized the value of well-characterized hematopoietic growth factor-dependent cell lines in advancing our understanding in the basic biology of myeloid cells.