By investigations on domestic and laboratory animals great progress was achieved in the clarification of the mechanism of action and of the metabolism of mycotoxins. The aflatoxins which inhibit the synthesis of RNA in the liver and which lead to a liver necrosis are all over the world of great importance as harmful substances for man and animals. The chronic intake in subtoxic doses has a tumour-evoking effect. The amatoxins of the green Amanita phalloides are also hepatotoxic: they inhibit the synthesis of messenger RNA in the hepatocytes. The ergot alcaloids unite with alpha-adrenoceptors and evoke an inhibition of the effectiveness of beta-adrenoreceptors: they effect a vasoconstriction, a contraction of the uterus and an inhibition of the prolactin secretion. The ochratoxins have an influence on the transcription and have a nephrotoxic and teratogenic effect. The trichothezene toxins inhibit the protein synthesis and already in a low dose decrease the formation of coagulation factors and of immunoglobulins. The zearalenon has an estrogen effect: it is bound to estrogen receptors and influences the transcription in the cell nuclei. Patulin has a tumour-evoking effect. Nutrients infested by fungi are to be excluded from the consumption by man.