The culture of Lactobacillus fermentum was isolated from the biogas substrate. The aim was to evaluate the efficiency of perorally applied L. fermentum additive to prevent metabolic diseases in the early lactation period of dairy cows. The experiment was performed in the early lactation group of a herd with 240 cows. The control and experimental group each consisted of 10 clinically healthy cows with normal concentration of β-hydroxybutyrate and glucose. On day 1–5 (D1–D5), the experimental cows received orally 150 ml of L. fermentum product of 8.1x105 CFU/ml. On D1, D2, D5 and D20, the rumen fluid samples were collected from all animals in both groups with an oral-ruminal probe once per day for detection of pH and concentration of volatile fatty acids, on D1, D5 and D20 – blood samples for biochemical analyses. The data were analyzed using Microsoft Excel. Results: Significant changes were observed in the concentration of the liver enzymes AST and GGT. On D1, in the experimental animals AST concentration 100.5±14.0 IU/L was higher than in control cows – 51.4±5.7 IU/L (p<0.05). On D20, AST was reduced significantly only in experimental cows. On D1, GGT concentration 31.5±6.91 IU/L was higher (p<0.05) in experimental animals than in control cows – 13.6±1.53 IU/L, but on D5, GGT concentration in experimental animals was reduced to 18.4±6.41 IU/L (p<0.05), and remained until D20. Conclusion: L. fermentum culture administered orally for five days improved the blood liver enzymes in cows, and the effect lasted for two weeks.