The enterotoxigenicity of 12 strains of coliform bacteria (Enterobacter cloacae, Klebsiella pneumoniae, and Escherichia coli) isolated from the gastrointestinal (GI) tract of persons with either acute diarrhea or tropical sprue and of 13 strains of the same species isolated from urine (GU) cultures was determined. Fractions of heat-labile (LT) and heat-stable (ST) toxins of each strain were separated by ultrafiltration, and the effect of graded concentrations (range, 100 microgram-10 pg/ml) on water transport was assessed by in vivo perfusion in the rat jejunum. Enterotoxigenic activity was defined as inducement of net secretion of water. All 12 of the GI strains and six of 13 GU strains elaborated enterotoxins, but there was a millionfold quantitative difference in the potency of the toxins produced. All of the GI strains produced one or both forms of toxin, which had a minimal effective concentration of as low as 0.1-10 ng/ml, whereas the GU strains produced toxins of weak or, rarely, of intermediate potency.