FIELD : medicine.SUBSTANCE : invention refers to medicine, namely to microbiology, infectious diseases, disinfectology, and can be used to study action of factors on biofilm and biofilm-forming ability of gram-positive microorganisms. Method for assessing composition of biofilms of gram-positive bacteria formed in polystyrene plates is carried out using 0.1?% aqueous solution of gentian violet with its extraction with alcohol and photometry of samples at wave length 560?nm, wherein coloring with gentian violet is carried out for 5 minutes with subsequent fixation of dye in bacterial cells by Lugol' s solution, colored products are dissolved with 70?% alcohol : matrix components for 1 minute, total biofilm for 15 minutes, after which biofilm composition is evaluated by formulas M=(OPbw/OP15)×100; Kb = 100-M, where M is matrix fraction, %; Kb – fraction of cellular component, %; OPbw – optical density of samples, when alcohol for dissolution of colored product is held for not more than 1 minute; OP15 is the optical density of the samples when the alcohol is dissolved for 15 minutes to dissolve the coloured product.EFFECT : method enables to differentiate the cellular component and biofilm matrix, while reducing the analysis time.1 cl, 3 ex