C5aR1 antagonist peptide, a biologically active linear peptide originating from the C-terminus of the chemokine complement fragment 5 anaphylatoxin (C5a), inhibits C5a binding and activity at human and rat C5a receptors. Overexpression of C5a is linked to various immunoinflammatory diseases, including arthritis, Alzheimer’s disease, cystic fibrosis, and systemic lupus erythematosus.