A set of cyclic peptide analogues of a selective CXCR4 antagonist FC131 [cyclo(-D-Tyr-Arg-Arg-Nal-Gly-)] were synthesized and bioevaluated. Using (E)-alkene and (Z)-fluoroalkene dipeptideisosteres for Arg-Arg and Arg-Nal substructures, indispensable or the partial contribution of the two peptide bonds to the CXCR4 antagonism and anti-HIV activity was demonstrated. FC131 and the analogues were shown