Analysis of DIDS-biotin binding to intact human erythrocytes. Washed normal human erythrocytes were reacted with DIDS-biotin reagent as described in Methods, labeled with streptavidin–Alexa Fluor 488, washed 3 times in buffered saline, and analyzed for bound Alexa Fluor 488 by flow cytometry. The intensity of the bound Alexa Fluor 488 fluorescence is plotted as a function of the concentration of DIDS-biotin reagent added. Fifty percent saturation was observed at 30 μM DIDS-biotin reagent.