Endogenous GpIb regulates the growth of hematopoietic cells. DAMI cells expressing GpIb were first isolated by affinity purification with solid-phase vWF and then selected by repeated rounds of fluorescence-activated cell sorting with the anti-GpIb antibody AN51. After selection, a population of cells designated DAMI GpIb (+) stains brighter with FITC-conjugated AN51. The inset shows that, when cells are counted 4 days after replating, DAMI GpIb (+) cell number is significantly decreased compared with DAMI GpIb (−) cells. (**P < .0001 compared with DAMI cells not expressing GpIb; n = 3.)