Advanced derivatives of the CXCR4 antagonist EPI-X4 have shown therapeutic efficacy upon topical administration in animal models of asthma and dermatitis. Here, we studied plasma stability of the EPI-X4 lead compounds WSC02 and JM#21, using mass spectrometry to monitor the chemical integrity of the peptides and a functional fluorescence-based assay that determines peptide function in a CXCR4-antibody competition assay. Although mass spectrometry revealed a very rapid disappearance of both peptides in human plasma within seconds, the functional assay revealed a significantly higher half-life of 9 minutes for WSC02 and 6 minutes for JM#21, respectively. Further analyses demonstrated that WSC02 and JM#21 interact with low molecular weight plasma components and serum albumin. Albumin binding is mediated by the formation of a disulfide bridge between Cys10 in the EPI-X4 peptides and Cys34 in albumin. These covalently linked albumin-peptide complexes have a higher stability in plasma as compared to the non-bound peptides and retain the ability to bind and antagonize CXCR4. Remarkably, chemically synthesized albumin-EPI-X4 conjugates coupled by non-breakable bonds have a drastically increased plasma stability of over 2 hours. Thus, covalent coupling of EPI-X4 to albumin in vitro before administration or in vivo post administration may significantly increase the pharmacokinetic properties of the new class of CXCR4 antagonists.