The reproduction rate of various microorganisms in milk may differ and depends mainly on the storage temperature. The initial degree of microbial contamination of the product, the presence and concentration of certain chemical substances also serve as factors that cause the development of dairy microorganisms. The research purpose is to identify the nature of the differential effects of antibiotics on the microbiological composition of raw milk, sourdoughs and fermented milk products. The authors studied physicochemical and microbiological indicators, the technological properties of the starter cultures obtained by milk fermentation with the use of antibiotics. According to the study results, during the life of lactic streptococcus the mass fraction of fat in the substrate was at a level of 0.36 to 0.42%, which is almost 2 times lower than in the starter cultures of Lactobacillus delbrueckii subsp. bulgaricus. This indicates that individual fat components and fractions are used to ensure the metabolic processes of Streptococcus salivarius subsp. thermophilus.