The pathogenic mechanisms of E. coli in diarrheal diseases were largely obscure up to now. The discovery of an enterotoxin which corresponded with the cholera enterotoxin in its mode of action and is also largely identical with this molecule immunologically provided essentially new aspects of the pathogenic importance of E. coli in diarrheal diseases. The detection of enterotoxin formation depends on the biological test model and presently it is still expensive and unsuitable for routine laboratory use. The serological identification of coli bacteria is meaningless for the detection of enterotoxin formation. Enterotoxic coli are more frequently found (12%) in the large group of hemolytic organisms. In 1978 we isolated enterotoxic coli from the stools of 12 infants under one year old with stubborn diarrhea.