Megakaryopoietic cells in the yolk sac and liver of mouse embryos were examined by electron microscopy. At 10 days' gestation, yolk sac vitelline vessels contained a few free megakaryopoietic cells. On the basis of the development of demarcation membranes and granules, yolk sac megakaryopoietic cells were classified into three types: YM1, YM2, and YM3. The YM1 cells, which comprised 75% of the yolk sac megakaryopoietic cells, had poorly developed demarcation membranes and few granules in the cytoplasm. The YM2 cells had a developed demarcation membrane system around the nucleus and comprised 24% of the yolk sac megakaryopoietic cells. The YM3 cells, the rarest type, had an eccentrically located nucleus and a cluster of demarcation membrane structures. In the liver of 11-day embryos, immature megakaryopoietic cells were present in both the sinusoidal lumen and the hepatic cords. Most of the hepatic megakaryopoietic cells of 11-day embryos had ultrastructural features similar to those of YM1 cells or intermediate between YM1 and YM2 cells.