Introduction. The increasing number of multidrug-resistant bacteria in the last decade has left clinicians with very few medication options, usually resulting in the use of more expensive treatments. The demand of new therapeutic approaches encourages the discovery of new natural products with possible antimicrobial activity. Aim of the study. Therefore, the aim of this study was to look for active substances that could be used as antibacterial agents. To achieve this objective, two different fractions (myxoxantophyll and phycocyanin) from Spirulina platensis were investigated. Myxoxanthophyll is a carotenoid glycoside yellowish pigment present in the photosynthetic apparatus of Arthrospira (Spirulina) platensis and phycocyanin is a protein complex, accessory pigment to chlorophyll also present in Spirulina platensis. Materials and methods. The cyanobacteria extracts were tested in vitro for their antibacterial proprieties against (Acinetobacter baumannii, Pseudomonas aeruginosa, Klebsiella pneumoniae, Staphylococcus aureus and coagulase-negative staphylococci) using macro dilution method Ericsson and Sheris. The Time-kill kinetics assay (CLSI M26) was used to study the bactericidal activity of the Spirulina platensis extracts against bacterial strains over the time. Results. By means of the broth macro dilution assay, it was found that microalga extracts possess pronounced antibacterial activity against Acinetobacter baumannii (MIC: 0,0275 mg/ml for myxoxanthophyll and 0,18 mg/ml for phycocyanin). In the case of coagulasenegative staphylococci the antimicrobial activity of Arthrospira platensis fractions was low. Gram-negative bacteria showed to be more sensitive to the action Spirulina platensis pigments than Gram-positive bacteria. Also, it was found that myxoxanthophyll possess bacteriostatic and bactericidal action at a lower concentration than the phycocyanin. At a concentration of 0,04 mg/ml myxoxantophyll could kill 100% bacteria in approximately 4 hours, and the timekill for phycocyanin was about 8 hours at the concentration 0,72 mg/ml. Conclusions. Further in vivo studies are required to investigate Spirulina platensis fractions potential toxic effects. In particular researches are needed to evaluate the use of control-release formulations in order to maintain the Arthrospira platensis pigments concentrations at antibacterial active doses.