Thrombopoietin receptor c-mpl is expressed on hematopoietic progenitors and cells of the megakaryocytic lineage. The c-mpl promoter may, therefore, be useful for directing the expression of transgenes. We tested whether a 2-kb genomic DNA fragment comprising the putative c-mpl regulatory elements and most of the 5'-untranslated region of mouse c-mpl is able to direct the expression of a reporter gene to hematopoietic cells in transgenic mice. As a reporter gene we used the human placental alkaline phosphatase (PLAP). In adult transgenic mice, PLAP expression was specifically detected in megakaryocytes and platelets. Embryos showed PLAP reporter gene expression already in the yolk sac at embryonic day 6.5 (E6.5) and in blood islands at E7.5. At E9.5, expression was found in blood vessels of the yolk sac and the embryo proper, followed by high levels of expression in the fetal liver at E11.5. Expression in E6.5 yolk sac is compatible with a function of c-mpl and its ligand, thrombopoietin, in the earliest stages of embryonic hematopoiesis.