The conducted studies confirmed the presence of a wide range of pathogens in the associations of pathogenic microbiocenoses in calves in diseases of the respiratory and gastrointestinal tract. Complex analysis of factors of microorganisms persistence in laboratory animals (AA and ALA), results of research of pathological material from calves with clinical symptoms of gastro-intestinal and respiratory diseases and antibiotic resistance of microflora isolated from them show that the Enterococcus and Streptococcus bacteria have the greatest pathogenic potential that confirms their leading place in the etiology structure of calf diseases. It is obvious from the results of the carried out researches that the level and coefficient of antibiotic sensitivity reflect the general level of "aggressiveness" of pathogens and characterize pathogenic potential of microorganisms. A study of the change of the antibiotic resistance coefficient showed that the highest index was in the Proteus genus, 0.91 + 0.03, while the lowest was in the Klebsiella genus, 0.84 + 0.05, which coincides with the general level of sensitivity of these microorganisms to the antibiotics under study.