The effect of the probiotic preparation Vetom 1 was studied on the quantitative indicators of microflora of gastrointestinal tract of pigs of the Large White breed kept at a pig farm. Vetom 1 is a powder for oral administration and rectal administration, containing dry bacterial mass of live spore-forming bacteria of Bacillus subtilis DSM 32424 strain. Based on the analogue principle, experimental and control groups of 10 animals in each were formed. Pigs of the experimental group were given Vetom 1 at a dose of 75 mg/kg of live body weight once a day every day for 15 days. When applied, the concentration of bifidobacteria increased. Vetom 1 did not affect the dynamics of the concentration of lactobacilli, enterobacteria and fungi in the gastrointestinal tract of pigs. Under the influence of the preparation, the correlation dependences between different populations of microorganisms of the gastrointestinal tract of animals changed. Without the use of the probiotic preparation, the concentrations of lactobacilli and bifidobacteria inversely correlated with each other with an average intensity. When using Vetom 1, the populations of these microorganisms in the gastrointestinal tract of pigs were synergistic in relation to each other and showed a positive correlation dependence of the average intensity. No correlation dependences were found between concentrations of enterobacteria and other studied microorganisms. Without the use of the probiotic preparation, concentrations of bifidobacteria and fungi inversely correlated with each other with a strong intensity. When Vetom 1 was used, the populations of these microorganisms in the gastrointestinal tract of pigs did not show either synergistic or antagonistic activity among themselves, since the confidence interval of the correlation coefficient equaled zero.