We have characterized the in vitro and in vivo toxic properties of cell sonic extracts from 22 animal and human clinical isolates of Escherichia coli that caused both necrosis in the rabbit skin and multinucleation in tissue cultures, two toxic properties previously reported as being specific for E. coli cytotoxic necrotizing factor (CNF). Two distinct toxic phenotypes were observed. Type 1, which was displayed by originally described CNF strains, was characterized by extensive multinucleation and rounding of cells in HeLa cell culture assays, moderate necrosis in the rabbit skin test, and absence of necrosis in the mouse footpad test. Type 2, which has recently been shown to be associated with E. coli Vir plasmid, was characterized by moderate multinucleation, by polymorphism and elongation of HeLa cells, and by an intense necrotic response in both the rabbit skin test and the mouse footpad test. The distinction between the two cytotoxins accounting for these effects (CNF 1 and CNF 2), together with their partial relatedness, was confirmed by seroneutralization studies of both cytopathic effects and necrosis in the rabbit skin test. In addition, type 2 extracts were more lethal in the mouse intraperitoneal test and induced a moderate, although not totally repetitive, fluid accumulation in the ileal loop test. The original toxic properties of these recently recognized categories of E. coli strains, together with their association with enteritis and septicemia, suggest that these strains may play a significant role in pathology.