The antioxidant properties of silibin complexes, the water-soluble form silibin dihemisuccinate (SDH), and the lipid-soluble form, silibin phosphatidylcholine complex known as IdB 1016, were evaluated by studying their abilities to react with the superoxide radical anion (O-2(-)), and the hydroxyl radical (OH.). In addition, their effect on pulmonary and hepatic microsomal lipid peroxidation had been investigated. Superoxide radicals were generated by the PMS-NADH system and measured by their ability to reduce NET. IC50 concentrations for the inhibition of the NET reduction by SDH and IdB 1016 were found to be 25 mu M and 316 mu M respectively. Both silibin complexes had an inhibitory effect on xanthine oxidase activity. SDH reacted rapidly with OH. radicals at approximately diffusion controlled rate and the rate constant was found to be (K = 8.2 x 10(9) M(-1) s(-1)); it appeared to chelate Fe2+ in solution.