The green microalga Haematococcus pluvialis has a specific biosynthetic activity at each growth phase, which includes synthesis, accumulation and transformation of carotene. Haematococcus pluvialis can accumulate up to 5% of the total cellular dry weight as astaxanthin, making it the best natural source of this high - value carotenoid pigment. The research was carried out to monitor the evolution of antioxidant activity of the active components of Haematococcus biomass during its life cycle. Inoculum of red aplanospores was used to initiate a new growth cycle by germinating with the formation of green motile flagellates. This early vegetative growth phase lasted for 9 days. In the next 4 days of the cultivation cycle, H. pluvialis formed spherical nonmotile palmelloid cells. Astaxanthin synthesis was artificially induced by strong illumination. The transition of astaxanthin accumulating palmella cells to red aplanospores lasted for 3 days. Ethanolic extracts were prepared from H. pluvialis biomass collected over the life cycle, to assess their radical scavenging capacity. The antioxidant tests were performed to reduce the radical cation ABTS•+ (2,2′-azinobis-(3-ethylbenzothiazoline-6-sulfonic acid) and the DPPH radical (1,1diphenyl-2-picrylhydrazyl) as oxidants. The results of ABTS assay revealed that the biosynthetic activity of the culture in the phase of green motile cells (days 1-9) showed stability of the antioxidant activity values. The ethanol extracts from the biomass of H. pluvialis in the palmelloid phase (days 10-13) and the red aplanospore phase (days 14 - 16) exhibited a linear increase in antioxidant activity with the maximum recorded on the last day corresponding to the stage of life cycle (Figure 1).figuraFig. 1. The antioxidant activity (ABTS, DPPH) of the ethanolic extracts from H. pluvialis biomass obtained during cultivation cycle (16 days). The DPPH assay also demonstrated a stability of values for the culture extracts obtained during green motile cells with a drastic reduction in antioxidant capacity in the last days of this vegetative growth phase. The reduction of β-carotene content in the cells affected the antiradicalic capacity. In H. pluvialis culture across the palmelloid phase (days 10 - 13) and the red aplanospore phase (days 14 16), the effect of a linear increase in antioxidant activity with the maximum recorded on the 15th day of the cultivation cycle was maintained. In the aplanospore stage with maximum accumulation of astaxanthin, the values of DPPH assay varied insignificantly – from 87% to 98% of inhibition. The stability of antioxidant values during green motile phase was the result of the active biosynthetic activity. The encystment process involved reorienting carotenoid pigments towards the formation of astaxanthin. Hence, antioxidant tests showed a linear increase in the free radical scavenging activity of the ethanolic extract obtained from palmella cells. The accumulation of astaxanthin as a result of culture exposure to the excessive illumination stabilized antioxidant values at a high level. In conclusion, both methods used to estimate the antioxidant activity of Haematococcus pluvialis algal extracts during its life cycle were informative.