The proof of the efficacy of a vaccine can be rendered only by field trials. Nevertheless, a number of in vitro and in vivo tests has to be employed before large-scale testing can be carried out. This particularly holds true about vaccines used to prevent intestinal infections, which were the center of our interest. Seven strains, four of them causing diarrhea in man, and three in animals, have been investigated as potential oral immunizing agents. The strains were made apathogenic by heating, or developing streptomycin-dependence, or by chemical agents. The efficacy of the vaccines was tested by feeding vervet monkeys with them, by determining their potency in the ligated rabbit and rat intestinal loop, by determining coproantibody formation and by estimating the circulating anti-enterotoxin as well as precipitating and bacteriolytic antibodies. Enterotoxin developed by human pathogens differed in its action from that of animalpathogenic strains. According to the penetrating ability of the strains, there was a difference between the circulating antibody evoking capability. Therefore in fundamental assay of the vaccines, the dose-response, the length of the administration of the immunizing agents, and their dosage had to be investigated before and after challenge with homologous and heterologous E. coli strains. It is suggested that coproantibody determination together with monitoring circulating antibodies after vaccination and after challenge be used to standardize these vaccines.