Cytotoxic necrotizing factor (CNF) toxins, isolated from certain Escherichia coli strains known to cause intestinal and extra intestinal infections, induce reorganization of the actin cytoskeleton and generate hyperploidy in adherent cell lines. We have examined the effect of CNF toxin on one of the few cell types that naturally increase nuclear DNA content, megakaryocytes. Our studies show that only hematopoietic cells capable of differentiating along the megakaryocyte lineage responded to the CNF2 toxin by becoming polyploid and by reorganizing actin. The K562, HEL, and CHRF-288–11 cell lines can be induced with phorbol ester to differentiate along the megakaryocyte lineage, and these cells also respond to the toxin with increased DNA content and actin cytoskeletal rearrangements. Interestingly, treatment of the K562 and HEL cell lines with CNF2 does not result in an increase in production of the megakaryocytic marker glycoprotein IIIa, unlike phorbol ester treatment. Conversely, two T-cell leukemic cell lines, CEM and Molt4, and the promyelocytic HL-60 cell line, which do not differentiate along the megakaryocyte lineage in response to phorbol myristate acetate, do not respond to CNF2, by increased expression of gpIIIa, increased nuclear DNA content, or actin reorganization. A potential target of these toxins, RhoA, is expressed by both megakaryocytic and nonmegakaryocytic cell lines, as shown by reverse transcription-polymerase chain reaction and Western blot. Although it is clear that the CNF toxins can affect a wide variety of adherent nonhematopoietic cell lines, we propose that the response to CNF, in terms of reorganizing actin structure and increase in DNA content in hematologic suspension cells, correlates with the capability of these target cells to differentiate along the megakaryocytic lineage.