We have identified an amino acid sequence, termed peptide 3, corresponding to amino acids 51-62 of the mature human monocyte chemoattractant protein-1 (MCP-1), which inhibits human mononuclear-cell and THP-1-cell migration induced by a wide range of chemokines. For example, peptide 3 inhibited MCP-1-induced THP-1 migration in a transwell assay with an ED50 of approx. 8 microM. Peptide 3 binds directly to THP-1 cells with an association constant of approx. 10 microM, and is therefore likely to be a direct receptor antagonist for CC and CXC chemokine receptors. By performing a structure-function analysis of this peptide, we have identified a sequence variant that shows an approx. 3-4-fold greater potency as an inhibitor of chemokine-induced migration [Leu4Ile11 peptide 3 (1-12)]. Furthermore, unlike peptide 3, which binds to the Duffy antigen receptor for chemokines on human erythrocytes with a similar affinity to the specific chemokine receptors on THP-1 cells, the Leu4Ile11 peptide 3 (1-12) sequence variant shows at least 20-fold greater selectivity for the specific receptors. Derivatives of Leu4Ile11 peptide 3 (1-12) are therefore the best candidates among the molecules we have investigated for use as a chemokine inhibitor in vivo.