The low-molecular-mass fraction of the soybean nodule cytosol contains Fe capable of catalyzing free radical production through Fenton chemistry. A large portion of the pool of catalytic Fe, measured as bleomycin-detectable Fe, was characterized as complexes of Fe with phenolic compounds of three classes: phenolic acids, cinnamic acids, and flavonoids. Many of these compounds, along with other phenolics present in legume tissues, were used for a systematic structure-activity relationship study. All phenolics tested were able to chelate Fe, as judged from their inhibitory effect on site-specific deoxyribose degradation (minus EDTA assay). However, only those having catechol, pyrogallol, or 3-hydroxy-4-carbonyl groupings were potent chelators and reductants of Fe 3 + at pH 5.5. The same phenolics promoted oxidative damage to DNA (bleomycin assay) and to deoxyribose (plus EDTA assay), but inhibited linolenic acid peroxidation by chelating and reducing Fe 3 + and by neutralizing lipid radicals. Also, phenolics having a pyrogallol nucleus attenuated the free radical-mediated inactivation of glutamine synthetase, which was used as a model system, by chelating Fe 2 + . It is reasoned that under the microaerobic (10-20 nM O 2 ) and acidic (pH 5.5-6.4) conditions prevailing in nodules, phenolics are likely to act primarily as anti-oxidants, decreasing oxidative damage to biomolecules.