Chemokine receptors play important roles in many physiological processes and are implicated in a wide range of human diseases including acute respiratory distress syndrome, allergic asthma, psoriasis, arthritis, multiple sclerosis, cancer, atherosclerosis and most notably AIDS. To enable the applications of chemokine ligands as probes of receptor biology and pharmacology, and inhibitors of diseases mediated by chemokine receptors, a major problem with the lack of receptor selectivity of these natural chemokines must be overcome. In this study, we have developed a chemical approach combining total protein synthesis and modular modifications to generate a new family of unnatural chemokines termed SMM-chemokines (which refer to synthetically and modularly modified chemokines) with designed receptor selectivity and affinity. A proof of the concept has been provided by applying this strategy to transform a very nonselective chemokine vMIP-II into new analogs with enhanced selectivity and potency for CXCR4 or CCR5, two principal coreceptors for HIV-1 entry. Such novel molecules have been shown subsequently to be valuable probes in gaining insights into receptor binding and signaling mechanisms, and as potent inhibitors to prevent HIV-1 entry and infection. These results strongly support the design concept of these SMM-chemokines and suggest that general applicability of this approach for studying and controlling other chemokine receptors and the diseases they mediate might be anticipated.