The effects of two dermatophytes (Microsporum gypseum and Trichophyton mentagrophytes) and four moulds (Scopulariopsis brevicaulis, Alternaria alternata, Geotrichum candidum and Penicillium spp.) on living keratinocyte cultures were examined in vitro using primary human keratinocytes. Rates of apoptosis of infected cells were determined using a colorimetric TUNEL system which detects the characteristic nuclear DNA fragmentation of apoptotic cells. The cytotoxicity of the individual fungi was tested by quantitatively measuring cytosolic enzyme lactate dehydrogenase, released upon cell lysis, in culture supernatants. Additionally, the cell structures within the infected keratinocytes in cultures were examined by scanning electron microscopy. All of the fungi exhibited high cytotoxicity, whereas the development of only the two dermatophytes and the mould Scopulariopsis brevicaulis resulted in distinctly increased apoptosis. Electron microscopy showed that all fungi studied caused similar alterations in the cell structure, with Microsporum gypseum being the most harmful. Increasing loss of cell adhesion as a consequence of a decreasing number of reticulating cell appendices and a reduced cell plasticity were the most evident alterations.