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Human RANTES (CCL5) and MIP-1α (CCL3) bind and activate several CC chemokine receptors. RANTES is a high-affinity ligand for CCR1 and CCR5, and it binds CCR3 with moderate affinity and CCR4 with low affinity. MIP-1α has similar binding characteristics to RANTES except that it does not bind to CCR3. Here we have generated a chimera of human MIP-1α and RANTES, called MIP/RANTES, consisting of the eight amino terminal residues of MIP-1α preceding the CC motif, and the remainder of the sequence is RANTES. The chimera is able to induce chemotaxis of human monocytes. MIP/RANTES has >100-fold reduction in binding to CCR1 and does not bind to CCR3 but retains full, functional binding to CCR5. It has equivalent affinity for CCR5 to MIP-1α and RANTES, binding with an IC₅₀ of 1.12 nM, and is able to mobilize calcium and induce endocytosis of CCR5 in PBMC in a manner equi-potent …