To attempt to clarify the mechanism of the action of mycotoxins under physiological conditions, the effect of simulated gastrointestinal conditions on the toxicity of Stachybotrys alternans was studied. Components, extracted into a simulated gastrointestinal system and into organic solvents were compared, regarding toxicity and chromatographic properties, with particular reference to a chromatographically characterized Stachybotrys-toxin found by us. It was established that, in addition to those toxic components which were soluble in organic solvents, some toxic, highly water soluble substances were extracted from the fungal culture by the gastrointestinal system. The total toxicity extracted by organic solvents was about fifteen times as much as the toxicity extracted by the simulated system, as measured by cell toxicity tests. All of the toxicity can be transferred from the fungus culture to the aqueous simulated system. This means that the simulated system altered the nature of the toxic components extractible with organic solvents.