The purpose of this study was to determine the effect of acetylation on the antioxidant activity of preparations of hydrolyzed plant proteins originating from two botanical sources; additionally, the preparations investigated differ in the method of their coagulation. Lentil and vetch seeds proteins were chemically modified with acetic anhydride, and precipitated using various coagulation agents such as: hydrochloric acid and polyelectrolytes (a cationic ‘Magnafloc LT-22S’ and an anionic ‘Magnafloc LT-27’). The protein preparations produced were hydrolyzed with tripsin, and lyophilised during a process of enzymatic hydrolysis. From the hydrolysates, free amino acids and peptides were extracted, and, next, their antioxidant activity was determined using a method of scavenging 1,1-diphenyl-2-picrylhydrazyl (DPPH˙) radicals. The study showed that the hydrolysis products of proteins contained in the leguminous plants under analysis are characterized by a good ability to quench a DPPH˙ radical. Both the modification performed and the methods of proteins coagulation impacted, to a very small degree, the properties under investigated. The anti-oxidative effect expressed as a per cent inhibition rate of inactivated radicals was 59.7 and 53.4% for the hydrolisates of vetch and lentil proteins, respectively, which were not treated chemically (control hydrolisates) and coagulated using only the hydrochloric acid. If compared to control samples, the hydrolysates obtained from acetylated proteins showed a higher ability to scavenge free radicals: as for lentils, this ability was by maximum 4% higher, and as for vetch – by 3% at the most. When high-molecular polyelectrolytes were applied as flocculation-coagulation agents, the properties under investigations increased by several per cent, and the proteins agglomeration, containing a flocculant ‘Magnafloc LT-22S’, caused a higher rise in the antioxidative activity compared to the effect achieved with the flocculant ‘Magnafloc LT- 27’. The key factors determining the scavenging effect of the DPPH˙ radical were: type of the plant species from which the protein preparation was produced, and, the time of reaction with radicals.