Abstract Flavonolignan silybin ( 1 ) present in the seeds of the milk thistle ( Silybum marianum ) is widely used in human therapy of liver dysfunctions and as a hepatoprotectant thanks to its dual function: it acts as a highly effective radical scavenger (antilipoperoxidant) and also as an antioxidant. Molecular mechanisms of antiradical action of 1 and even functional groups responsible for this activity are not well known so far. Silybin forms during in vitro reaction with stable radicals (e.g., DPPH) or with enzyme laccase ( Trametes pubescens ) complex mixtures of oligomeric and polymeric products whose structural analysis is virtually impossible. Methylation of 7-OH in 1 yields under laccase-mediated oxidation C–O and C–C dimers in the ratio ca. 1:2.5. Using this approach, the hydroxyl groups responsible for antiradical activity of silybin (20-OH group) were determined and the molecular mechanism of the E-ring antiradical activity was explained.