['Megakaryocyte polyploidization results from the lack of cytoplasmic separation while the nucleus keeps dividing.', 'To investigate the role of actin in the megakaryocyte polyploidization, three human cell lines with megakaryocytic properties (DAMI, HEL and K562) were incubated in the presence of cytochalasin B, an inhibitor of actin polymerization. These data were then compared with normal megakaryocytes.', 'Compared with control conditions, cells cultured in the presence of cytochalasin B revealed an augmentation of cell size and ploidy and an arrest of cell proliferation. The expression of platelet membrane glycoproteins Ib, IIb/IIIa, IIIa and thrombospondin and transferrin receptors was augmented after treatment with cytochalasin B. Physiologically, the role of actin in inducing polyploidization could be related to an imbalance between G- and F-actins. To test this hypothesis, we measured G-, F- and total actin in cytochalasin B-treated cells. Actin was found to be increased significantly in cytochalasin B-treated DAMI and HEL cell lines. In contrast, the G/F-actin ratio was not affected by cytochalasin B. To confirm these actin changes in physiological megakaryocytopoiesis, G- and F-actin contents were then estimated in normal megakaryocytes. The G- and F-actin contents of megakaryocytes from eight normal patients exponentially decreased from 2 to 128n, whereas the total actin content per cell kept increasing. The G/F ratio was unaffected.'] Are induction and enhancement of normal human megakaryocyte polyploidization concomitant with perturbation in the actin metabolism?