In most eukaryotic cells the regular alternation of chromosome reduplication and cell division is controlled by interdependent relationships which prevent progression to the next cell-cycle phase unless the preceding phase has been completed. Megakaryocytes become polyploid by allowing many rounds of DNA replication without completion of intervening mitoses. To assess the role of cell-cycle dependencies in megakaryocytopoiesis we examined human cell lines which express megakaryocytic features for their ability to continue DNA synthesis and undergo polyploidization in the presence of mitotic poisons. Treatment of HEL cells with colcemid blocked cell division but not cellular DNA synthesis. DNA content distributions of cells treated with colcemid for 48 h showed a marked increase in the proportion of polyploid cells (57.6% +/- 9.9%, n = 16), an increase in cellular size and nuclear lobation. Identical effects were observed in HEL cells treated with colchicine, nocodazole or taxol but not with the inactive compound lumicolchicine. Induction of polyploidization by antimicrotubule agents was also observed in the megakaryoblastic cell lines MEG-01, DAMI and UT-7 but not in the T-cell line MOLT-4 or the promyelocytic cell line HL-60. These results suggest that dependency of DNA replication on completion of the previous mitosis is suppressed in the megakaryocytic lineage.