Introduction. The formation of a microbial biofilm in implant-associated infection after arthroplasty of large joints reduces the informative value of traditional microbiological diagnostic methods and limits the range of effective antimicrobial drugs. When prescribing etiotropic therapy for periprosthetic joint infection, it is necessary to take into account not only the antibacterial effect of the drug, but also its effect on biofilm formation. Ciprofloxacin therapy may be a risk factor for the development of biofilm periprosthetic infection caused by multidrug-resistant staphylococcal strains.The aim of research was to study the effect of sub-inhibiting and therapeutic doses of ciprofloxacin on biofilm formation by Staphylococcus epidermidis strains isolated from implant-associated infection.Materials and methods. The authors studied the effect of various concentrations of ciprofloxacin on 15 strains of St. epidermidis isolated from 83 patients with deep periprosthetic joint infection after primary knee arthroplasty, treated at NIITON SSMU in 2018-2019. The effect of the calculated concentrations of ciprofloxacin on the planktonic culture, forming and preformed biofilms was investigated. Biofilm simulation was performed according to the method described by G.D. Christensen under in vitro conditions with determination of the optical density of alcohol eluates of gentian violet in polystyrene microplates.Results. It was demonstrated that ciprofloxacin in a dose 0.01 g/ml inhibits the growth of planktonic forms by 50% and statistically significantly (p = 0.001) stimulates formation of microbial biofilms as compared to the control without antibiotic addition. Concentration of ciprofloxacin equal 0.03 g/ml inhibits the growth of planktonic forms by 90%, statistically significantly (p = 0.001) stimulates formation of biofilms and activates further increase in the mass of pre-formed microbial biofilms. An increase in the concentration of ciprofloxacin to 0.05 g/ml completely inhibits the growth of planktonic forms and statistically significantly stimulates further growth of preformed biofilms.The use of ciprofloxacin at concentrations equal 1-3 g/ml statistically significantly (p = 0.001) inhibits the formation of microbial biofilms, but does not affect the preformed biofilm.Conclusions. Thus, there has been found a dose-dependent effect of ciprofloxacin towards clinical strains of St. epidermidis: subinhibitory and therapeutic concentrations of the drug have a stimulating effect on the formation and further increase in the mass of the preformed microbial biofilms. This fact must be taken into account when prescribing etiotropic therapy for implant-associated complications following large joint replacements.