Various extracts of Japanese quince (Chaenomeles japonica) seeds obtained using organic solvents were studied for their polyphenol content and antiradical activity. It was established that petroleum ether, hexane, ethyl acetate, acetone, as well as toluene and chloroform extracts in comparison to synthetic antioxidant BHT demonstrate better (or comparable) activity against DPPH. Methods for detoxification of seeds, meals and press-cakes are proposed. Phenolic composition of different extracts (80% ethanol, 70% acetone), both acid and alkali hydrolysates of seeds, as well as seed oil methanol/water extract were analysed by means of HPLC: chlorogenic acid was found for the first time in seed extract; protocatechuic acid predominated in all extracts. The content of other major phenolic acids was detected; it was found that seed oil contains syringic acid. It was determined that Japanese quince seeds contain almost 10 times more -tocopherol than barley grain. Due to the presence of alpha-tocopherol and phenolic compounds seed oil and lipophilic extracts of seeds could serve as antioxidants.