Probiotic feed supplement for dogs containing 60 parts by weight of probiotic species Lactobacillus fermentum AD1-CCM 7421 with density 1.0 x 10exp9 of nucleuses expressed in colony forming units KTJ/g and 40 parts by weight of inuline. The species Lactobacillus fermentum AD1-CCM 7421 was isolated from ecosystem of dogs and shows advantageous properties modifying intestinal micro flora and at the same time optimizing effect on level of multiple biochemical parameters of blood of dogs. The probiotic feed supplement preventative applied to dogs prevents diseases of gastrointestinal tract caused by lesion of intestinal microbial balance. It acts as supporting preparation at the therapy of the above mentioned diseases.Probiotic feed supplement for dogs containing 60 parts by weight of probiotic species Lactobacillus fermentum AD1-CCM 7421 with density 1.0 x 10exp9 of nucleuses expressed in colony forming units KTJ/g and 40 parts by weight of inuline. The species Lactobacillus fermentum AD1-CCM 7421 was isolated from ecosystem of dogs and shows advantageous properties modifying intestinal micro flora and at the same time optimizing effect on level of multiple biochemical parameters of blood of dogs. The probiotic feed supplement preventative applied to dogs prevents diseases of gastrointestinal tract caused by lesion of intestinal microbial balance. It acts as supporting preparation at the therapy of the above mentioned diseases.