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The study of the influence of the non-specific external factor -- the acidity of the medium, revealed the dependence of the film forming processes on the intensity of the factor and time of it action. The aim of the research was to study an influence of acidity of nutrient media on biofilm formation of isolates of S. aureus and S. epidermidis. Methods. Isolates of S. aureus (n=7) and S. epidermidis (n=20) were inoculated on media with different pH (from 4.0 to 8.0) and cultivated 72 h. After 24, 48 and 72 h of cultivation the quantity of living cells was determined. Results. It has been shown that the maximal increase of cell number in films of S. aureus and S. epidermidis occurred in media with neutral pH, as well as the growth rates were closer to the maximum when the film was grown at pH 6 and 8. Thus, at neutral pH, the maximal cell growth took place for 72 h, when the number of cells in S. aureus film was 9.52±8.56 lg CFU / ml and S. epidermidis was 9.29±8.07 lg CFU / ml. At pH 4 and 5, the intensity of the growth of the culture was the smallest: the maximal cell count in the film did not exceed 5.5 lg CFU / ml. Conclusion. The strains of S. aureus were more resistant to the influence of acidity of medium, compare to S. epidermidis isolates. [ABSTRACT FROM AUTHOR]