The enterotoxigenicity of strains of Klebsiella pneumoniae, Enterobacter cloacae, and Escherichia coli, which represented the predominant coliform species isolated from the jejunum of 12 patients with tropical sprue and 5 with the blind-loop syndrome, was quantitatively assessed in terms of the ability of toxin preparations to induce water secretion as assayed by in-vivo perfusion in the rat jejunum. All 12 patients with sprue harboured 1 or more highly toxigenic strains--14 of the 16 strains isolated from this group produced heat-labile and/or heat-stable toxins which were as potent as toxins derived from strains isolated from persons with acute diarrhoea and documented as toxigenic. None of the 9 strains isolated from patients with the blind-loop syndrome produced potent toxins. This difference between the coliform bacteria in sprue and the blind-loop syndrome probably accounts, at least partly, for the different intestinal response in these two disorders to contamination by these organisms.