Live cells and cell-free culture supernates of 50 strains of Aeromonas hydrophila isolated from diarrhoeic and healthy human faeces, drinking water, sewage, the river Ganges and faeces from domestic animals caused accumulation of fluid in ligated ileal loops of adult rabbits. The amount of fluid produced was comparable to that of a toxigenic strain of Vibrio cholerae. Three of the strains gave positive reactions only after two passages in ileal loops of rabbits. Inocula of about 10(3) viable cells and 0-25 ml of culture supernate caused fluid accumulation in the loops. The enterotoxic factor was inactivated at 60 degrees C for 20 min. and 65 degrees C for 10 min., was precipitated with ammonium sulphate and was non-dialysable; these results indicate the protein nature of the enterotoxin. An inoculum of 40 microgram of crude toxin caused as much fluid accumulation as larger inocula. The only histopathological change in the loops was depletion of mucus from the goblet cells.