Murine megakaryocytes generated in a liquid culture system containing pokeweed-mitogen-stimulated spleen-cell-conditioned medium were classified into four groups: heterogeneous colonies, homogeneous colonies, two-cell colonies, and solitary megakaryocytes. The ploidy distribution and ultrastructural cytoplasmic maturation were compared among the four groups. The mean ploidy of heterogeneous colonies was low and remained unchanged during the culture. In contrast, an intensive increase in DNA content of homogeneous colonies, two-cell colonies, and solitary megakaryocytes was observed. In ultrastructural analyses, type-I megakaryocytes (early immature megakaryocytes) were predominant in heterogeneous colonies. Type-II megakaryocytes (immature megakaryocytes) increased in homogeneous colonies and two-cell colonies. Type-III megakaryocytes (mature megakaryocytes) increased in two-cell colonies and solitary megakaryocytes. In the liquid culture system, megakaryocytes from two-cell colonies and solitary megakaryocytes could mature into platelet-producing megakaryocytes, whereas megakaryocytes from heterogeneous colonies and homogeneous colonies remained immature.