GpIb-mediated growth arrest is partially dependent on extracellular vWF. The relative number of CHO/β/IX cells growing in serum-free medium 4 days after replating or serum-free CHO In/β/IX cells after 4 days of continuous treatments with muristerone is smallest when they are grown on vWF. Ristocetin (15 μg/mL) was added to every culture well. In the absence of vWF, a significant decrease in cell number is still observed in CHO cells expressing GpIb. (**P < .0001 compared with V/β/IX cells growing in serum-free medium; *P < .001 compared with V/β/IX cells growing in serum-free medium; n = 3.)