These studies were carried out to detect the presence of mycotoxin fungi and detoxification of toxins in various kinds of grain and foodstuffs in Korea. The experiments were divided into three parts: mycologic, toxicologic and electron microscopic study. The resuIts were summarized as follows: 1. From the 210 various, samples(local grains, 150 samples; rice-cakes, 50 samples; meju, 10 samples), 1,205 colonies or fungi were isolated. In 1,127 of 1,205 colonies, it was possible to identify 21 genera. Among the identified strains, the predominant genera were Aspergillus sp.(28.5%), Penicillium sp.(27.1%) and Mucor sp.(7.8%). 2. In cytotoxicity test on HeLa cells, 25 strains showed severe toxic effects among the 240 strains of experiments. 3. In histopathologic test on mice, 21 strains showed severe toxic effect to mouse liver cells among the 240 tested strains. 4. In electron microscopic studies of HeLa cells and mouse liver cells from animal which had been treated with crude toxin, the liver cells showed the cytoplasmic change: dillatation or vacuolization of endoplasmic reticulum, swelling of mitochondria and disappearance of mitochondrial cristae, increased number of lipid and glycogen particles. Nucleus and nucelar envelope alterations, were also noted. 5. In the detoxification study with ultraviolet irradiation, 90% of the toxic substances were denatured by the ultraviolet light irradiation for 24 hours. 6. As a mass screening, the cytotoxicity test of HeLa cells and histopathologic study of mice liver cells treated with culture filtrates, employed and feasible to detect mycotoxin producing fungi.