Arthrospira platensis (spirulina) belongs to microalgae and has been consumed by humans for millennia mainly because of its high content of high quality proteins, vitamins, minerals and other bioactive compounds. Due to the known beneficial effects of lactic acid fermentation on the shelf life, functionality and sensory properties, fresh biomass of A. platensis microalgae was subjected to lactic acid fermentation by two lactic acid bacteria species - Lactobacillus plantarum and Lactobacillus brevis. By varying inoculum size (1 %, 5 % (v / v)), nutrients added (dH2O, solution of glucose and yeast extract, 0.9 and 3 % (w/v) NaCl) and fermentation time (24, 48, 72 h) the optimal fermentation conditions according to the measured parameters (pH, lactic acid bacteria concentration and lactic acid concentration) were selected. Lactic acid fermentation proceeded optimal with 1 % inoculum and addition of 0,9 % (w/v) NaCl as a liquid. Due to the constant values between 24 and 72 h, 24 h was selected as the optimal fermentation time. We also found that native lactic acid bacteria are already present in the microalgal biomass, which independently perform the fermentation to some extent, but they do not make a major contribution when using starter culture. Furthermore, the antioxidant activity of water and ethanol extracts of non-fermented and fermented microalgae paste using yeast Saccharomyces cerevisiae was tested. Compared to non-fermented paste, the antioxidant activity of ethanol paste extracts increased after fermentation, but decreased or did not change in water extracts, depending on the concentration of the extract and the starter culture used. The A. platensis microalgae proved to be a good sole substrate for lactic acid fermentation where their antioxidative properties tested on the cell model also changed.