IFNγ significantly decreases ¹²⁵I-rhSCF–specific binding to ECFC in a dose-dependent manner. Day-5 ECFCs were incubated at 37°C with various concentrations of rhIFNγ between 0 and 5,000 U/mL for 24 hours and specific binding of 0.5 nmol/L ¹²⁵I-rhSCF was determined. (★) P < .03 between control cells and cells treated with 250 or 500 U/mL rhIFNγ; (★★) P < .01 between the control cells and cells treated with 2,500 or 5,000 U/mL rhIFNγ. At 24 hours, ECFC purity was 42% ± 4% for control cells and 43% ± 4% for 5,000 U/mL rhIFNγ-treated cells with no significant changes in total cell concentrations. Data are expressed as means ± SD of triplicates.