EPI-X4, an endogenous peptide fragment of human serum albumin (amino acids 408-423), serves as an antagonist to the chemokine (C-X-C motif) receptor 4 (CXCR4). It effectively prevents calcium mobilization and receptor internalization triggered by chemokine (C-X-C motif) ligand 12 (CXCL12) in HEK293 cells with CXCR4 expression, at concentrations between 0.8 and 1,000 µM. Moreover, EPI-X4 hampers the CXCL12-driven migration of both Jurkat T cells and human CD34+ hematopoietic stem cells. In an acute allergic airway hypereosinophilia mouse model, EPI-X4 intrinsically decreases Cxcr4-dependent inflammatory cell infiltration in airways at a dose of 16 µmol/kg. Additionally, increased urinary EPI-X4 levels, inversely related to glomerular filtration rate (GFR), are observed in chronic kidney disease patients, indicating its potential biomarker role.