Downregulation of iodinated chemokine binding to monocytes by F peptide. Monocytes were preincubated with F peptide at the indicated concentrations for 60 minutes at 37°C. The cells were then incubated with ¹²⁵I-MIP-1β (A) or¹²⁵I-SDF-1 (B) at room temperature for 40 minutes and pelleted through a sucrose cushion. The cell-associated radioactivity was determined. Unlabeled MIP-1β or SDF-1 at 1,000-fold excess was used to define the level of maximal direct competition by native ligand. The percentage of inhibition of specific chemokine binding is shown in parentheses. *P < .05 compared with binding to cells incubated with medium only.