This thesis focuses on the impact of the fungi Alternaria alternata (A. alternata) and its spores of contaminated food on the gastrointestinal tract after the ingestion. A. alternata is a worldwide occurring mould and able to already infest crop and therefore also affects the products made thereof. However, a later contamination of food with this fungi is also possible. The consumption of these contaminated products represents a risk for human’s and animal’s health due to the fact that fungi from the species Alternaria are capable of producing numerous mycotoxins. Alternariol (AOH), Alternariol-monomethyl ether (AME), Altenuene and Isoaltenuene (ALT and iALT), Altertoxin-1, 2 and 3 (ATX-1, 2 and 3) and Tenuazonic acid (TeA) are among the most important mycotoxins, which are synthesised by A. alternata. In vitro studies with human, colorectal carcinoma cells (Caco-2 cells) reveal these secondary metabolites to have mutagenic, cytotoxic and in some cases even carcinogenic properties. Stimulation of cells with these toxins revealed an inhibition of the topoisomerases, single- and double strand breaks of the DNA, a reduced proliferation rate and an accumulation of cells in the G2/M-phase of the cell cycle. Furthermore, a greater production of proinflammatory cytokines and IgA and an inhibition of the protein synthesis was detected. Although these in vitro studies clearly demonstrate Alternaria toxins to have damaging effects on Caco-2 cells, there is a lack of in vivo studies. A single in vivo mouse experiment reported the development of precancerous lesions in the oesophagus after feeding contaminated feed. This finding correlates with the observation of a greater incidence of oesophageal cancer in several regions of Asia and Africa, where higher amounts of Alternaria mycotoxins can be found. In general, there is an urgent need for further studies to evaluate the impact of Alternaria toxins on the integrity and functionality of the gastrointestinal tract.