The problem of treatment of acute digestive disorders with signs of diarrhea in calves remains relevant and important, as there are no sufficiently effective drugs, including antimicrobial effects. The low effectiveness of antibiotics and other chemotherapeutic agents and therapies in the vast majority of farms dictates the need to find new drugs. The purpose of our work was to study the significance of opportunistic microorganisms in the etiology of calving diarrhea. The research was conducted at farms of Lviv and Volyn regions. The incidence of newborn calves with acute digestive disorders with signs of diarrhea ranged from 36.8 to 100%, and mortality ranged from 17.4 to 30.8%. Bacteriological study was subjected to patmaterial from 20 dead calves at the age of 1–7 days. In this case, 88 cultures have been isolated from the internal organs, mesenteric lymph nodes, bone marrow, brain, walls and intestines. According to the culture – morphological properties, the cultures were classified into the following species: Escherichia coli, Proteus vulgaris, Pseudomonas aeruginosa, Diplococcus capsulatus, Clostridium perfringens. In the bacteriological study of all microorganisms isolated - Diplococcus capsulatus 29.5%, Escherichia coli 30.7%, Pseudomonas aeruginosa 19.3%, Clostridium perfringens 11.4%, Proteus vulgaris 9.1%. Out of the 14 households, the causes of the disease of calves with acute gastrointestinal disorders in 35.7% of cases were certain opportunistic pathogens (Escherichia coli, Diplococcus capsulatus, Pseudomonas aeruginosa, Clostridium perfringens, Proteus vulgaris). In most farms (64.28%), the disease was caused by association (2, 3 and more) types of microbes. The pathogenic properties of the isolated cultures were tested on white mice. In determining the pathogenicity of isolated cultures from dead calves on white mice, the most pathogenic were microorganisms of the species Pseudomonas aeruginosa. Experimental mice (42.0%) died within 6–12 hours after infection. Highly pathogenic were mixed cultures of E. coli, diplococci, Pseudomonas aeruginosa, which killed 58.0% of infected mice 24–48 hours after infection. This indicates an increase in the pathogenic properties of microorganisms in the association. In order not to allow the passage of conditionally pathogenic microflora, it is necessary to adhere to the sanitary regime in dispensaries and maternity departments, and the terms of disinfection with variable sectional retention.