Disclosed are polypeptide analogues of human C5a which are C5a receptor antagonists that exhibit substantially no analphylatoxin or agonist activity, and derivatives of the analogues, and dimeric forms of the analogues or derivatives. DNA molecules encoding the polypeptides and methods of making the analogues are also provided. Pharmaceutical formulations containing a C5a analogue, are used therapeutically in the treatment of C5a-mediated diseases and inflammatory conditions in mammals, and prophylactically to prevent or reduce inflammation caused by an event which causes inflammation or aggravates an existing inflammatory condition, respectively. Further disclosed are antibodies specific to the C5a analogues, derivatives thereof, and dimers of the analogues and derivatives which exhibit substantially no cross-reactivity with human C5a. The antibodies are used to detect or quantify circulating C5a analogue or derivative, as well as to modify, e.g., neutralize, the activity of the C5a receptor antagonist in vivo.