Cianobacterium Arthrospira platensis (spirulina) is unanimously considered a complex and valuable non-conventional source of an interesting array of biologically active compounds. Highvalue compounds from this organism have been put to assorted uses as cosmeceuticals, nutraceuticals, and as functional foods. From the technological point of view, it presents advantages due to low nutritional demand, low cost of compound extraction and dry biomass. Spirulina has a high adaptability and can be cultivated in various nutrient conditions. The use of various chemical compounds, and more recently nanoparticles, either in the composition of cultivation media or administered at different growth stages of spirulina culture, can induce changes in both productivity and the content of functional components of biomass. During the metabolism of compounds, the antioxidant activity of the biomass also changes, which is due to the formation and/or annihilation of free radicals. Changes in the antioxidant activity of spirulina biomass (ethanolic and hydric extracts), cultivated in the presence of 5 nm Cu nanoparticles with polyethylene glycol (PEG) coating, have been established, supplemented to culture medium in concentration range from 0.05 μM to 2.5 μM. Antioxidant activity was determined on the basis of the reduction of the radical cation ABTS•+ (2,2′azinobis-(3-ethylbenzothiazoline-6-sulfonic acid). ABTS assay revealed the antioxidant activity of ethanol extracts in the range of about 9 - 11% in the case of low doses of copper nanoparticles (Figure 1).figuraFig. 1. Antioxidant activity (ABTS, % inhibition) of ethanolic and hydric extracts obtained from spirulina biomass, grown in the presence of CuNPs with polyethylene glycol coating. Applying high concentrations (1.0 - 2.5 μM), the antioxidant activity of ethanolic extracts decreased by about 16 - 45%. In the case of hydric extracts, low concentrations (0.05 - 0.5 μM) did not significantly alter antioxidant activity, with the exception of 0.1 μM concentration, ABTS test values increased with 14%. High concentrations (1.0 - 2.5 μM) induced an increase of about 2963% in the antioxidant activity values. The data generated in this study can serve as background for further selection of nanoparticle concentrations based on the dynamics of the antioxidant capacity of biomass and/or each type of extract – criteria that will be taken into account when including polyethylene glycol coated nanoparticles in the technological schemes intended to obtain raw materials of bionanoparticles and functional compounds from spirulina.