The conversion of Zearalenone by some strains of microorganisms was investigated. When the selective strains of Rhodotorula sp., Arthrobacter sp., Saccharomyces sp., and Candida sp. were incubated by shaking or standing at 28 degrees C for 72 h with an alcoholic solution of Zearalenone as the substrate at a concentration of 2-10 mg/ml ethanol (50-100 micrograms/ml medium), it was readily converted to give Zearalenols, consisting either mainly of the alpha-isomer (e.g. 96% in case of Rhodotorula sp. and 84% in case of Arthrobacter sp. as determined by HPLC) or beta-isomer (e.g. 91% and 92% in Saccharomyces sp. and Candida sp., respectively). The structure of the product was confirmed by 13C-NMR, MS and HPLC.