The inhibitory activity of new isoflavonoids on 5-lipoxygenase of porcine leukocytes was investigated. Isoflavans (I) proved to be stronger inhibitors than isoflavones (II). The isoflavans containing ortho-hydroxy groups in ring A showed the lowest Ki values (0.8-50 microM). In comparison, isoflavans with meta-dihydroxy groups exhibited Ki values higher than 150 microM. The effect of commercial antioxidants was tested also on porcine 5-lipoxygenase. Butylated hydroxyanisole (Ki: 25 microM) and butylated hydroxytoluene (Ki: 55 microM) revealed moderate inhibitory activity, whereas L-ascorbic acid, L-ascorbyl palmitate, dl-alpha-tocopherol and n-propyl gallate showed weak inhibitory activities (Ki: 100-260 microM).