The chemokine RANTES (regulated upon activation, normal T-cell expressed and secreted) is a natural ligand of CCR5, one of the major HIV-1 coreceptors. It is secreted as part of the immune response to human immunodeficiency virus 1 (HIV-1) and inhibits infection by CCR5-dependent (R5) HIV-1 isolates. We have investigated the interaction of RANTES with several peptides derived from the extracellular domains of CCR5 by heteronuclear NMR spectroscopy in aqueous solution. We show that a peptide comprising the first 25 amino acid residues of the CCR5 N-terminal domain and sulfated at the Y10 and Y14 side-chains binds with micromolar affinity exclusively to the monomeric form of RANTES. In contrast to the tight binding of the sulfated peptide, the affinity of the same peptide in non-sulfated form was reduced by more than two orders of magnitude. Peptides derived from the CCR5 extracellular loops ECL1, ECL2 and ECL3 showed only very moderate and mostly non-specific binding. Chemical shift mapping of the interaction of the sulfated N-terminal peptide reveals a contiguous binding surface on RANTES, which comprises amino acid residues of the first beta-strand, the N-loop, the fourth beta-strand and the turns around residues 30 and 40. This binding surface largely overlaps with the dimer interface and is strongly positively charged, providing a rationale for the exclusive binding of the monomer to the peptide and the requirement of the negative sulfate groups at the Y10 and Y14 side-chains. The binding surface also largely overlaps with the segments that were identified previously as crucial for HIV blockade by peptide scanning and mutagenesis studies. These data offer new insights into the structure-function relation of the RANTES-CCR5 interaction and may be helpful for the design of novel HIV-1 inhibitors.