CCR5, chemokine receptor and HIV coreceptor, is sulfated in its extracellular N-terminal domain, with sulfation known to be important for binding of native chemokines. Using a panel of CCR5 N-terminal sulfopeptides, we identified anti-CCR5 monoclonal antibodies showing distinct preferences for CCR5 sulfation patterns. Testing these mAbs in cross-competition binding assays and under conditions that modulate CCR5 tyrosine sulfation, we showed that incomplete sulfated CCR5 variants exist at the cell surface. We observed that native RANTES/CCL5 engages a small fraction of cell surface CCR5 that can be bound by 5P12-RANTES, a highly potent anti-HIV chemokine analog, showing that this fraction can be increased or decreased under experimental conditions that modulate CCR5 tyrosine sulfation. We propose that incompletely sulfated CCR5 variants contribute to formation of CCR5 low-affinity subpopulations for native chemokines, and that 5P12-RANTES binds to these subpopulations with high affinity because of additional compensatory binding contacts via its engineered N-terminal region