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Aeromonas sp., grown in tryptone soya broth supplemented with yeast extract, 0.6%, pH 7.5, and incubated with agitation at 100 oscillations/min for 15 h at 37°C produced optimal amounts of β-haemolysin and cytotonic enterotoxin. More prolonged incubation resulted in the loss of enterotoxic activity and anion exchange chromatographic analysis indicated the presence of a moiety capable of brerking down the toxin. Anion exchange fast protein liquid chromatography resulted in a single peak of haemolytic activity and two peaks with enterotoxic activity. The cytotonic enterotoxin was purified from the fraction most active in the infant mouse assay; the second peak, which did not cross-react immunologically, may represent a second cytotonic enterotoxi. Neither peak was observed in the chromatographic fractions of filtrates from strains devoid of activity in the infant mouse assay. Purified enterotoxin, estimated to have a mol. wt of 15kDa by SDS-PAGE, caused fluid accumulation in the infant mouse assay, was non-haemolytic to rabbit erythrocytes, caused an increase in cAMP activity in tissue culture cells and did not cross-react immunologically with components of cholera toxin or the whole toxin. Purified β-haemolysin had an estimated mol. wt of 55 kDa, lysed rabbit erythrocytes and did not cause fluid accumulation in the infant mouse test.