Antigenic relationships between enterotoxins of Vibrio cholerae and Escherichia Coli were investigated. In tests in the rabbit intestinal loop, high dilutions of equine antiserum to choleragenoid neutralized heat-labile enterotoxin (LT) from porcine and from human enteropathogenic E. Coli (EEC). This neutralizing effect was removed by absorption of the serum with choleragenoid. Sera with high titers of activity against E. Coli LT had no neutralizing effect on choleragen, the enterotoxin of V. cholerae 569B. E. Coli LT was neutralized in rabbit gut loop tests by antisera to E. Coli strains that produce both LT and heat-stable enterotoxin (ST) but not by antisera against strains that produce ST only. LT produced by different serotypes of porcine EEC were highly related antigenically to one another, and antigenic relationships were demonstrated between LT from different serogroups of human EEC and between LT from human and porcine strains of EEC.