The aim of this study was to determine the influence of spontaneous fermentation and that with the use of probiotic bacteria and yeast on aflatoxin B₁ concentration and the microflora pattern during fermentation. The probiotic preparation used contained bacteria resistant to gastric juice and bile: Lactobacillus paracasei LOCK 0920, Lactobacillus brevis LOCK 0944, Lactobacillus plantarum LOCK 0945, as well as live yeasts Saccharomyces cerevisiae LOCK 0140 of high fermenting capacity. After 6-h fermentation with the probiotic, in feed mixture with a low concentration of aflatoxin B₁ (1 mg/kg), the amount of aflatoxin B₁ decreased by 55%. In the case of a high concentration (5 mg/kg) the decrease in aflatoxin B₁ was about 39%. This tendency was sustained during the following hours of incubation (12^th and 24^th h). The application of probiotic bacteria and yeasts resulted in the reduction of aerobic spore forming bacteria.