Mycotoxins can cause various toxic effects in humans. Acute and chronic respiratory diseases were reported after inhalation of organic dust containing toxigenic moulds and mycotoxins, respectively. To gain first insights into health effects from airborne exposure to these compounds, five toxigenic airborne moulds of the genera Aspergillus and Penicillium collected at composting plants and eight reference mycotoxins were tested for cytotoxicity in four established cell lines as a surrogate of tissues known or suspected to be targets of toxic effects of mycotoxins. The known mycotoxins sterigmatocystin, fumagillin, verruculogen, penitrem A, and roquefortine C were detected in extracts of the moulds. All five extracts caused serious toxic effects in the cell lines. Sterigmatocystin caused a 80-fold higher toxicity in the A-549 lung cell line compared to Hep-G2 liver cells indicating a specific susceptibility of A-549 to this agent. Since only a minor part of the toxic effects of the extracts in A-549 cells and--to a lesser extent--in the other cell lines could be explained by contents of the identified mycotoxins, the presence of additional mycotoxins or other toxic principles is assumed in the mould extracts. However, the detected mycotoxins in the mould extracts and their distinctive cytotoxicity support the hypothesis that mycotoxins may be involved in the aetiology of lung diseases due to the inhalation of organic dust.