Twenty-five strains of enterotoxigenic (tox + ) Escherichia coli isolated in India, Bangladesh, and the U.S.A. were shown to produce vascular permeability factor (PF) activity as well as diarrheagenic activity. Sixteen strains of non-enterotoxigenic E. coli were found to be PF negative. The PF response elicited by tox + E. coli isolated in the U.S.A. from cases of infantile diarrhea was qualitatively different and more difficult to demonstrate. However, this problem was surmounted by assaying 25-fold concentrates of the crude enterotoxin of these strains. PF activity of various strains of tox + E. coli was neutralized by anti-PF antiserum prepared against the enterotoxin of strain H-10407. This anti-PF antiserum also neutralized PF activity of Vibrio cholerae enterotoxin (choleragen). Antiserum containing antibody specific for purified choleragen neutralized E. coli PF activity. These and other results indicate that the PF activity of E. coli is a function of the diarrheagenic enterotoxin as is known to be the case with choleragen. We conclude that the PF assay can be employed for the identification of tox + E. coli and for the detection of antitoxic antibody.