After immunohistochemical staining the size and maturational stage of fetal megakaryocytes from 20 human fetuses of 12 to 21 weeks' gestation were compared with those from normal adults. The mean diameter of fetal megakaryocytes was 14.0 microns when stained with antiglycoprotein IIb (Tab) and 15.2 microns when stained with antifactor VIII, which was significantly smaller than adult megakaryocytes, which were 18.4 microns when stained with Tab and 20.6 microns when stained with factor VIII. The proportion of immature (stage II) cells was higher--and the proportion of mature (stage IV) cells was lower--in the fetal tissue than in the adult. The smaller size and shift to a less mature population indicated that there were differences at the non-mitotic phase of the development of megakaryocytes in the fetus. Such differences were probably associated with quantitative and qualitative platelet abnormalities in newborn infants. Understanding the physiology and regulation of megakaryocytopoiesis in fetuses and newborn infants will be invaluable in determining the pathophysiology of platelet dysfunction in the newborn.