In 1999-2001 the investigations on mycological state of stored and sold fruit, berries, and vegetables grown in Lithuania and imported from other countries were performed. The samples of foodstuff were taken from storehouses, various supermarkets, and market places. Such ecological conditions lead to a rapid spreading of micromycetes and contamination of other articles of food stored and sold nearby. On fresh fruit and berries the development of microorganisms is slow. However, microorganisms penetrate into internal tissues of berries and fruit, thus becoming difficult to notice visually. Some microorganisms, especially micromycetes of some species belonging to the Penicillium Link, Aspergillus Mich. ex Fr., and other genera, are able to produce secondary metabolites (mycotoxins) of various compositions that are toxic to plants, animals, and humans. Therefore, the ability of micromycetes to synthesise and excrete toxic secondary metabolites was examined. Considering this issue, 393 micromycete strains ascribed to 54 genera and 176 species were tested. 46 strains were identified as active producers of toxic substances and were selected for further examinations. Most of them belonged to the Penicillium, Aspergillus and Fusarium genera. Their detection frequency on the investigated berries, fruit, and vegetables was determined, and the impact upon warm-blooded animals (BALB/c mice) was tested. Significant changes of the internal organs and blood composition were found in mice infected with toxic micromycetes. In conclusion, it was evidenced that more than 10% of micromycete strains developing on incorrectly-preserved fruit, berries and vegetables, produce toxic secondary metabolites that pose a potential health hazard for people eating or handling the foodstuff.