Seeds of leguminous plants can be a potential source of biologically active antioxidants – polyphenols. The objective of this paper was to optimize the extractive system, as well as to determine the antioxidant activity and the flavonoid composition in some selected seeds of leguminous plants. A mixture of acetone and water, their ratio being 7:3, was used for the extraction. In the extracts, the content of polyphenols in total (converted into gallic acid) was determined. The highest contents of polyphenols in total were noted in the ‘Red Kidney’ coloured beans (5.35 mg/g s.s.), white beans (1.08 mg/g s.s.), and ‘Ramrod’ peas (0.86 mg/g s.s.). The antioxidant activity of samples investigated was investigated using ABTS^•+ and DPPH^• methods. The results of the two methods evidenced that the coloured bean of the ‘Red Kidney’ cultivar had a higher antioxidant activity than the white bean and pea (in the case of ABTS^•+ – 8 times higher, and of DPPH^• – 6 times higher). A significant quantity of tannins was noted only in seeds of the coloured beans. The occurrence of ferulic acid, quercitin, luteolin, and kaempferol (determined using HPLC) were found in extracts of the selected varieties of seeds. The quercitin (245 mg/g s.s.) predominated in the ‘Red Kidney’ coloured beans, the luteolin (82 mg/g s.s.) – in the ‘Ja Karłowy’ white beans, and the kaempferol (85 mg/g s.s.) – in the ‘Ramrod’ peas. It was proved that the coloured beans were a richer source of polyphenols than the white beans and peas.