The diffusible haemolysin of Escherichia coli is highly cytotoxic to chick embryo cell cultures, but has no effect on monkey kidney and mouse embryo cell cultures. The cytotoxic effects are specific, as they can be neutralised by specific antisera. Culture fluids from non-haemolytic strains of E. coli are non-cytotoxic. The haemolytic activity is thermolabile, but cytotoxicity is thermostable. The antibodies prepared against heated or unheated haemolysins prevent cytotoxicity, but haemolytic activity is neutralised only by antisera prepared against unheated haemolysin. Fraction A of haemolysin is non-haemolytic and non-cytotoxic, whereas fraction B is non-haemolytic but is cytotoxic.