A liquid culture system is described for murine megakaryocyte progenitor cells (CFU-M) in the presence of pokeweed-mitogen-stimulated spleen-cell conditioned medium. There were dose-related responses between the number of CFU-M developed and the number of cells cultured and the dosage of conditioned medium in this liquid culture system. Murine CFU-M were abundantly cloned in this system an the plating efficiency was similar in comparison with that in a plasma clot system. The acetylcholinesterase-positive colonies (more than 4 acetylcholinesterase-positive cells) were clearly seen on day 3 of culture, and they reached a maximum (60.5 +/- 10.7/2 x 10(5) cells) on day 7 of culture. Ultrastructural analyses of megakaryocytic maturation in this system showed that a few megakaryocytes produced platelets that were released in the culture medium on day 5 of culture. This liquid culture system is suitable for the study of the dynamic process of the megakaryocyte-platelet system.

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