Clostridium difficile produces one diarrhoeogenic toxin designed A, and one cytopathogenic toxin designed B. Toxin A was purified in a four-step-fractionation procedure. In the last purification step the toxin was separated by elution with galactose from an agarose gel. The purified toxin A induced a clear and watery hypersecretion in intestinal loops of mouse, while mixtures of toxin A and B induced a haemorrhagic secretion. At an ED50 value for the purified toxin A of 0.5 microgram there was a brief, optimal hypersecretion after four hours. Like the fluid secretion induced by cholera toxin, that induced by toxin A could be inhibited by chlorpromazine or by depletion of intestinal bile. In contrast to cholera toxin, however, toxin A did not activate intestinal adenylate cyclase--at least not permanently. Antisera which neutralized cholera toxin did not neutralize toxin A, and vice versa.