μ ₃-oxotrinuclear Iron(III) glycinates have been manifested as modulators of antioxidant activity of Porphyridium cruentum biomass, a red microalga. The coordination compounds with composition [Fe3O(Gly)6(H2O)3](NO3)n·xH2O (n=1, x=3 (1); n=7, x= 3.5(2)) [1] were supplemented to the cultivation medium of microalga in various concentrations. There were prepared the aqueous and ethanolic extracts with concentration of ethyl alcohol 55% and 96% from microalgal biomass. The antioxidant activity of aqueous extract from biomass of Porphyridium cruentum, grown on the medium with the addition of glycinates in concentrations of 10 and 20 mg/L, showed inhibition of the radical ABTS (2,2-azinobis-3-ethylbenzothiazoline6-sulfonic acid) [2] exceeding control samples with values 23,4-30,7% (figure). The ethanol extract 96% manifested a higher antioxidant capacity, when microalga was cultivated on the medium with the addition of 40 mg/L of 1 complex and 10 mg/L of 2 complex. The extracts from biomass of Porphyridium cruentum, grown on medium with the addition of iron(III) glycinates, showed an increased capacity to reduce phosphomolybdic reagent compared to control biomass [3]. In the case of monoanionic glycinate 1, the increasing of antioxidant activity of both extracts was observed at low concentrations of compound (10 and 20 mg/L), the values being 15,9-27,8%. Complex 2 caused a significant increase of reduction capacity of the extracts with 23,3-77,0%. The ethanolic extract from microalgal biomass, grown on the medium with the addition of 10 mg/L of 2, showed a capacity to reduce phosphomolybdic reagent equivalent with 108,6 mg Trolox for g biomass, which can be regarded as a good result for a performance antioxidant. The results of monitoring the antioxidant activity of extracts, using reaction reducing Folin-Ciocalteu reagent [4], also demonstrated the possibility of increasing the antioxidant activity of Porphyridium biomass. The highest capacity of reducing reagent was observed in the case of aqueous extracts, which in the best experimental variants (20 mg/L compound 2), showed an antioxidant activity by 2,2-3,8 times as high as the activity of the extract from control biomass. Therefore, coordination compounds of iron(III) with glycinate can be applied in cultivation technologies of Porphyridium cruentum in order to obtain antioxidant preparations.