Fourteen patients who underwent Charnley or Howse total hip replacement using CMW bone cement were studied. Mean values of C4 and C3 determined before the induction of anaesthesia and insertion of bone cement and also after this procedure showed no significant difference and were within the normal range. Furthermore, no evidence of C3 breakdown products was found. It was therefore concluded that neither the general anaesthetic technique nor the insertion of bone cement activated the classical or alternative complement pathways and such a mechanism was not responsible for the hypotension which occurred shortly after the insertion of bone cement in 10 cases studied.