Escherichia coli strains isolated from extraintestinal infections of humans possess a constellation of phenotypes not usually found in random fecal isolates, enteropathogenic strains, or the laboratory strain E. coli K12. The phenotypes found more commonly inextraintestinal strains include hemolysin production, the biosynthesis of colicin V, and the hemagglutination of human erythrocytes in the presence of d-mannose (HAh). Hemolysin is assumed to be a cytotoxic factor, colicin V is assumed to interfere with host defense mechanisms, and HAh is assumed to play a role in specific tissue adherence. In addition, ≥50% of E. coli strains from extraintestinal infections kill allantoically inoculated 13-day-old chick embryos. Some (20%) of the fecal E. coli also kill chick embryos, but E. coli K12 is innocuous in this virulence model. The plasmids for hemolysin and colicin V production have been transmitted to E. coli K12 derivatives but are not sufficient to convert laboratory strains to a form virulent for the chick embryo.