Staphylococcus epidermidis, a coagulase-negative Staphylococcus, is the most important pathogen responsible for chronic nosocomial infections. These bacteria produce extracellular slime and form biofilms on various biotic and abiotic surfaces. Bacterial biofilms are very resistant to standard antimicrobial therapy and difficult to eradicate, so it is important to search for new more effective anti-biofilm agents, for example in the group of natural substances. The aim of the study was to examine the activity of two ionophores-salinomycin and monensin against clinical S. epidermidis strains, using MIC/MBC method and biofilm formation inhibition assay. Bacterial strains were tested also for slime production using Congo Red Agar. Both tested ionophore antibiotics showed the highest activity against planktonic bacteria of clinical as well as standard S. epidermidis strains and effectively inhibited the formation of bacterial biofilm.