TPO-induced increases in actin assembly in primary megakaryocytic and HEL cells can be inhibited by WT. Cells were either unstimulated or stimulated for 2 hours with 100 ng/mL TPO in the absence or presence of 100 nmol/L WT. After stimulation, cells were permeabilized, labeled with BODIPY fluorescein-conjugated phallacidin, and fluorescence assessed by FACScan as described in Materials and Methods. All results in a given experiment are normalized to the mean channel fluorescence of unstimulated HEL or primary CD34-derived cells, which was given an arbitrary value of 1. Relative F-actin contents greater than 1 indicate an increase in actin polymerization relative to unstimulated cells.