Expression of the main nuclear protooncogenes during terminal megakaryocyte (MK) differentiation is poorly understood. Because previous results have suggested that c-fos and c-jun protooncogenes are expressed in human leukemic cell lines induced to undergo megakaryocytic differentiation, we have analyzed the expression of these two protooncogenes in normal MK. Studies were performed, by in situ hybridization and immunofluorescence, on human MK obtained either directly from bone marrow or from culture of MK progenitors. c-fos and c-jun transcripts were detected in most cultured or fresh marrow MK from adult donors. Expression was much higher in cytologically immature than in mature MK whereas no expression was detected in the most mature MK. c-fos and c-jun expression increased dramatically with MK size. In cultured fetal MK, which all remained small in size, c-fos mRNA was present but at a low level. The c-fos-encoded protein (P62fos) was easily detectable in the great majority of MK. We directly demonstrated that the level of P62fos expression was correlated to MK ploidy by flow cytometry using a three-color staining technique. The involvement of serum and growth factors in the induction of P62fos in MK was studied. Whereas a 3-h serum deprivation resulted in the disappearance of P62fos in MK, several growth factors such as granulocyte-macrophage colony-stimulating factor (GM-CSF), interleukin 3 (IL-3), interleukin 6 (IL-6), interleukin 7 (IL-7), leukemia inhibitory factor (LIF), and transforming growth factor beta (TGF-beta), as well as normal or aplastic serum, were able to reinduce its expression within 2 h. In conclusion, our results suggest that c-jun and c-fos may play a role in the transduction of signals by several growth factors during terminal MK differentiation.