Sendai virus is haemolytic against erythrocytes from various species. Pretreatment of the virus with antibody and complement (C) resulted in a sixfold increase in the haemolysis. The viral haemolysin (HL) could be inactivated by heating the virus at 37 degrees C for 5 h. HL-inactivated virus became haemolytic again when pre-treated with antibody and C. This acquired haemolytic capacity corresponded to the enhanced haemolysis shown by intact virus. It is concluded that the enhanced haemolytic activity is due to a C-mediated indirect haemolysis and is not dependent on the fusion process. The possibility of cell damage in vivo from virus-antibody-C complexes is discussed.