Summary. The RBC haemolysis resulting from Clostridium welchii septicaemia is generally thought to be caused by the lecithinase (α toxin) produced by that bacterium. A woman admitted to this hospital suffering from a Clostridium septicaemia presented us with the opportunity to study whether structural and compositional changes in her RBC membrane could account for the observed haemolysis. Previous studies showed that treatment of RBC membranes with phospholipase C (α toxin), obtained from Clostridium welchii, produces vast changes in both the phospholipid structure and composition, without any detectable alteration in the protein composition or structure. The RBC membranes, prepared from the patient's blood, exhibited little or no change in phospholipid composition and structure, when compared with normal cells; however, both the protein composition and structure appeared drastically altered. These results, therefore, suggest that the haemolysis observed during Clostridium welchii septicaemia is not the result of a lecithinase (α toxin), but may be due to the proteolytic action of one or more of the other toxins secreted by this bacterium.