Objective. To evaluate and compare the antioxidant activity displayed by seaweed H. opuntia and B.triquetrum using different experimental in vitro antioxidant assessment models.Material and Methods.The following techniques are utilized: DPPH, Reducing power, inhibition oflipid peroxidation and inhibition of haemolysis.Results. B. triquetrum: DPPH; IC50=1.15 ± 0.06, Reducing power (concentration 128 mg/mL), OD=2.798,inhibition of lipid peroxidation; IC50=5.09± 0.25 and inhibition of haemolysis; with 12 mg/mL; 35 %. H.opuntia: DPPH; IC50=12.34 ± 0.30 mg/mL, reducing power; OD=0.800, inhibition of lipid peroxidation;IC50=1.25± 0.31 mg/mL, inhibition of lipid peroxidation; IC50=1.25± 0.31 mg/mL and inhibition of haemolysis;82%.Conclusion. It was demonstrated that B.triquetrum extract was more effective than H. opuntia in reducingpower and DPPH assays while H.opuntia was more effective in inhibition of lipid peroxidation in ratbrain homogenates and the inhibition of red blood cell (RBC) haemolysis induced by AAPH. We discusssome aspects about their possible mechanisms of action.