A lesion-inducing factor (LIF) of virulent avian strains ofEscherichia coliwas investigated. Liver lesions were produced by concentrated dialyzed filtrates of broth cultures grown at 37 C in a synthetic medium. LIF activity was present in washed cell walls and in extracts from intact cells prepared by a procedure using 1 MNaCl. Extraction with 5 × 10−3 MEDTA alone or in combination with 1 MNaCl also resulted in preparations with satisfactory LIF activity. Preparations obtained by hot water or phenol–water extraction possessed less LIF activity on a weight basis. LIF activity was shown in extracts from three different serogroups ofE.coli.