Extracellular regulators of human megakaryocyte development are becoming better defined. How these regulators function at the subcellular and, in particular, the molecular levels remains almost completely unknown. The recent development of molecular micromethodologies such as in situ hybridization, the polymerase chain reaction, and the use of antisense oligodeoxynucleotides now make such studies possible in normal cells. We therefore examined the effect of several recombinant human hematopoietic growth factors and the maturation agonist phorbol myristate acetate on the expression of selected growth-regulated and maturation/function-related genes. We also examined the role of the c-myb proto-oncogene in regulating megakaryocyte proliferative activity and ploidy development. Our results demonstrate that growth factors have complex time and concentration effects on gene expression in morphologically recognizable human megakaryocytes. They also suggest that a more complete understanding of normal megakaryocyte development at the molecular level will soon be possible.