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 M.