Megakaryocyte colonies can be grown in culture from human blood cells and fetal liver cells in plasma clot containing erythropoietin. Megakaryocyte progenitors were found in a fraction of mononuclear cells isolated by Ficoll density gradient centrifugation from adult, neonatal, and fetal blood. Megakaryocytes were identified by their morphology and particularly by their polylobulated nucleus when examined by light microscopy. The megakaryocytic nature of large cells was clearly confirmed by the presence of platelet peroxidase, demarcation membranes, and alpha-granules detected by electron microscopy; in addition mature small megakaryocytes were recognized. Megakaryocyte colonies were seen after 9 days of culture and consisted of 2 to 20 cells. The colonies were pure or mixed with the burst erythroblasts. The mixed colonies were numerous in fetal and neonatal cultures, while pure megakaryocyte colonies were seen three times more frequently in those from adult blood. The total number of colonies was also much lower in adult cultures. In colonies derived from neonatal and fetal cells, megakaryocytes often reached a more complete maturation than in those from the adults, proceeding as far as platelet shedding. This study demonstrates for the first time that a megakaryocyte committed cell present in human blood can develop megakaryocyte colonies in culture.