Edema disease principle (EDP) extracted from porcine Escherichia coli strains was shown to be cytotoxic for Vero cells. This biological effect was thermolabile and specifically neutralized by anti-EDP serum. The EDP did not cross-neutralize with antiheat-labile enterotoxin or anti-Vero toxin sera. Parallel studies conducted in mice indicated that the same entity may be responsible for the in vivo and in vitro effects. The results of these experiments provide an effective basis for the first in vitro EDP serologic test.