Isolated guinea pig megakaryocytes were maintained in liquid cultures for up to 4 days. Megakaryocytes were incubated in siliconized glass vials in Dulbecco's Modified Eagle Medium with 5%-10% guinea pig serum and 2.3% bovine serum albumin. Cultured megakaryocytes did not adhere to glass vials and were almost entirely recovered by aspiration. No reproduction or cell division of megakaryocytes occurred. A small decline in viability occurred promptly on placing the freshly isolated cells in culture medium and could be attributed to reexposure to calcium. On incubation there was little further cell death. Up to 2 days in culture the megakaryocytes remained morphologically intact and appeared similar to megakaryocytes in situ. Megakaryocytes matured in culture with a loss of cytoplasmic basophilia, an increase in granule content, and progressive changes in nuclear configuration. The most mature megakaryocytes developed pseudopod formation but large-scale platelet liberation was not seen. The ability to culture megakaryocytes in vitro will allow more extensive biochemical and physiologic studies of this cell than previously possible.