Isoflavones and their glycosides were isolated from kudzu root, and formononetin‐7‐O‐glucoside (ononin) was identified from the plant for the first time. Among isolated compounds, the antioxidant potency of 5 major compounds was determined using (1) 2,2‐diphenyl‐1‐picrylhydrazyl free‐radical scavenging assay, (2) thiocyanate assay in the linoleic acid model system, and (3) lipoxygenase inhibition assay. The study indicates that 5 compounds act as free‐radical scavengers and inhibit both linoleic acid peroxidation and lipoxygenase activity. The 3 different assays for determining antioxidant activity revealed that activity in one test did not necessarily correlate with activity in another test. Arachidonic acid release in the intact HL‐29 cancer cell, biochanin A, showed strong inhibition of about 70%, at a concentration of 50 μ.M.