The authors have discovered a highly selective CXCR4 antagonist, T22 ([Tyr^5,12,Lys⁷]-polyphemusin II), and its shortened potent analogs, T140 and TC14012, which strongly inhibit the T-cell line-tropic HIV-1 (X4-HIV-1) infection through their specific binding to a chemokine receptor, CXCR4. CXCR4 is a major coreceptor (second receptor) for the entry of X4-HIV-1 into T-cells. These peptides have been found through the structure-activity relationship (SAR) study on tachyplesins and polyphemusins, which function as self-defense peptides of horseshoe crabs with immature immune systems. T140 and TC14012 showed the highest level of anti-HIV activity and antagonism of target cell entry by X4-HIV-1 among all the CXCR4 antagonists that have been reported to date. Additionally, bifunctional anti-HIV agents based on the specific CXCR4 antagonists (T140 analogs)-3'-azido-3'-deoxythymidine (AZT) conjugation have been synthesized and evaluated, since T140 analogs can possibly work as a carrier of AZT targeting T-cells due to their specific affinity for CXCR4 on T-cells. T22 have two disulfide bonds and a Trp residue in the molecule. In connection with this study, novel facile and side-reaction-free methodologies for disulfide bond formation have been established for the increase of the efficiency of SAR studies. Furthermore, the completely stereocontrolled synthetic process for a couple of (E)-alkene dipeptide isosteres starting from L-amino acid has been established in order to facilitate nonpeptidylation studies on peptide-lead candidates. In this review, the authors wish to summarize our recent research on the development of specific antagonists against the HIV second receptor CXCR4, involving studies on the establishment of efficient methodologies for the facile synthesis of peptides and peptide mimetics.