IP-10 is a member of the chemokine family of cytokines and is induced in a variety of cells in response to interferon gamma and lipopolysaccharide. The self-aggregation common to many chemokines, including IP-10, has hindered the identification of a specific IP-10 receptor. Using an IP-10 alkaline phosphatase fusion protein that fortuitously blocks this self-aggregation, we have identified an IP-10 binding site on a variety of cells including endothelial, epithelial, and hematopoietic cells. This binding site has a Kd of 25 nM, is inhibited by recombinant murine or human IP-10, and is dependent on the presence of cell surface heparan sulfate proteoglycans (HSPG). This conclusion is based on the findings that IP-10 binding to cells is: (a) inhibited by heparin and heparan sulfate; (b) sensitive to a 1 M NaCl wash; (c) eliminated by treatment with heparinase and trypsin; and (d) absent on mutant CHO cells that do not express cell surface HSPG. Platelet factor 4 (PF4), but not IL-8, monocyte chemoattractant protein-1, RANTES, monocyte inflammatory protein (MIP)-1 alpha, or MIP-1 beta, can compete effectively with IP-10 for binding to the cell surface. Furthermore, IP-10 shares with PF4 the ability to inhibit endothelial cell proliferation (IC50 = 150 nM). These studies demonstrate specificity in the interaction of chemokines and HSPG, and they define IP-10 and PF4 as a distinct subset of chemokines sharing an HSPG-binding site and angiostatic properties.